I've been in the habit for years of building my own PCs, partly for the cost savings, partly because I'm a geek, and partly (mostly), because I've found the best way to tune a system exactly to my liking is to pick only and exactly the parts that I need. Once I discovered Linux a couple of years ago, I had the perfect match for my hobby. I'll lay out on these pages what I've learned by trial and error, what to look for in a DIY computer, and how to best mix-and-match according to your desires and budget.
For starters, the key to building your own system is to find the best sources for parts. Computer Shopper is probably the DIY bible, crammed with mail--order ads from companies selling parts. I prefer the face-to-face purchase, myself. Most of my buying takes place at the ubiquitous "computer flea markets" that take place every month or so in most major metropolitan areas. In Greater Boston (my stomping grounds), there are two major shows put on; KGP and Northern. These are held in halls around the metro area, and there's one every few weeks within driving distance. Typically, many vendors attend all the shows in a given area.
Most vendors are pretty reliable in my area (your mileage may vary), and are usually willing to play the deal game. This is where your objectives come into play.
Fortunately, Linux isn't too picky about the hardware it runs on--just about any old CPU will suffice. The major areas of concern are in deciding whether or not to use IDE or SCSI drives and what type of video card to install. Assuming that you will use a standard Linux distribution, the screaming video card that plays Doom at warp speed under DOS may not be supported by Xfree 86. For instance, the immensely popular Trident 9440 VGA chipset only recently became supported by X, though it shipped with Windows 95and OS/2 drivers. Anyhow, in making these decisions, I have a simple checklist:
The answers to these questions should help determine what you need to purchase. First off, let's cover processor type/speed and RAM. Linux is somewhat more efficient in its consumption of system resources than DOS (or pretty much any other Intel OS), so you may not necessarily need the screaming Pentium 200 that you need for your Windows 95 system. In the Pentium class processors, currently the 100 and 133 MHz Pentiums are the best values in bang-for-the-buck specs. Both chips are well under $200, and the 100 MHz processor is close to $100. I tend to suggest those processors that operate on a 66 MHz motherboard bus clock (like the above two chips--the P166 and P200 are also in that category). Generally speaking, the faster clock speed of the Pentium 120 and 150 are offset by the slower 60 MHz bus and higher price. A good PCI motherboard to accompany the chip costs about $100 to $150. Stick with boards that use the Intel chipset for safest results, though I have had good luck with other vendors.
If you don't need to go Pentium class, there are some bargains out there. AMD makes some very good 486 chips, running at up to 120 MHz. This is about equivalent in horsepower to the original Pentiums, but without the math errors. The most recent system I built uses a hybrid motherboard (one older VL-bus slot, 4 PCI slots), and has an AMD 5x86-133 chip. This processor is kind of a cross between a 486 and a Pentium, and competes very well with the Pentium Overdrive upgrades that Intel sells to 486 owners. The 5x86's performance is roughly on a par with a Pentium-90, and motherboard/processor combined cost roughly $100 (as opposed to about $150 for the Overdrive itself).
Basically; you can factor out the price/performance scale like this:
|486 (66-120MHz)||VL bus||low-decent $75-$100|
|5x86VL PCI or both||low-end Pentium||$100-$120|
|Pentium 100PCI only||Good for multiple OS||$200-$250|
|Pentium 133PCI only||Fast Linux, games'll rock||$300-$350|
|Pentium 166PCI only||Wow, that's fast!||$475-$550|
|Pentium 200PCI only||Ahead ludicrous speed, cap'n!||$700+|
|Pentium ProPCI only||If you need it, buy it built...|
When you buy the motherboard, there is another factor that has recently become worth considering: what form factor do you use? Newer Pentium and Pentium Pro-based motherboards are often available in the ATX form factor. The board is easier to service, and the cases are easier to take apart. ATX boards and cases are a little tougher to find, but there is no real cost difference between ATX and the traditional Baby-AT form factor, so you may wish to consider the ATX alternative at purchase time.
If you buy the motherboard and case from the same vendor, often they will mount it in the case for you. If you do it ourself, be careful to make sure that the power supply is properly connected, both to the motherboard and to the power switch. Power supplies have two keyed connectors attaching them to the motherboard. It is difficuly, but not impossible, to wire them wrong (I have a friend who did), so make sure the black wires on the power leads are touching on the inside: ADD DIAGRAM HERE
The motherboard also should be connected to the case with at least two spacers that screw down in addition to all the plastic posts that will be in the case kit. This insures that cards fit properly, and keeps the board stable.
Besides the processor/motherboard combination, there are other performance issues, of course. RAM is finally cheap enough that you should buy at least 16 MB worth (about $100 at current street prices). Linux will run OK in 8 MB (and even 4 MB is OK for text-based work), but why scrimp there when it costs so little to do it right? If you buy from a show vendor, make sure they test it in front of you. Any reputable vendor has their own RAM tester. Generally, there is no real price difference between conventional fast-page RAM and the slightly faster EDO variety, but make sure your motherboard uses the type of RAM you're buying. Most better motherboards will happily auto-detect the type of RAM you use and configure themselves correctly. But you can't mix, so make sure you only install one type, whatever that is. Newer Pentium chipsets support the newer SDRAM, which promises even more speed. I have not yet tried it in a system, so I cannot tell you whether or not that is so. Buy 32 MB if you can afford it--you won't regret it.
There's also the IDE-SCSI decision. IDE interfaces are built into most modern motherboards, so it costs nothing extra. And IDE hard drives are a little cheaper, and IDE CD-ROMs are fast, cheap (under $80 for a 4x drive), and easy to set up. But the controllers only support four devices total (two ports, with two devices each), and each IDE channel is only as fast as the slowest device on it (meaning you really can only have two hard drives, and the CD-ROM has to go on channel 2). And modern multitasking OSs like Linux can't get their best performance out of IDE. But it's cheap and easy. SCSI is higher performance, and has none of IDE's restrictions (up to 7 devices per controller, no transfer rate limit beyond the adapter's), but the controller will set you back $70 (for a basic Adaptec 1522) to $200 (a PCI controller) plus. The drives don't cost much more, and you can only get the highest performance drives in SCSI versions. SCSI CD-ROM drives are a little harder to find, but the basic 4x drive will only cost you about $125. And SCSI tape drives (you were planning to back up your data, weren'>t you?), are much easier to install and operate than their non-SCSI counterparts (faster, too). I'd say the decision is one to be made after you've priced the rest of the system out. If you can afford it, SCSI will make for a better system in the long run.
The video card decision is also an important one. The critical part of this decision is picking a card that uses a chipset (the actual brains of the card) which is supported by XFree86, the standard Linux XWindows with most distributions. A few distributions (Caldera, Red Hat) ship with commercial X implementations that have a little more flexibility in video support. I find S3-based video cards to be the most universally supported--the S3 driver in XFree86 is very solid and works even with most of the generic, no-name video cards on the market. The S3 cards generally have a large (about 1.5" x 1.5") chip with the S3 brand name prominently displayed on it. Diamond and Number Nine make extensive use of S3 chips in their video card lines, to name a couple of brands. Among other SVGA chipset makers, Cirrus and Trident are also well-supported. Only the latest X versions include support for the popular Trident 9440 chips, so be careful before buying a video card with that chipset. XFree86 includes a very complete readme with the status of support for most video cards/chipsets, so consult it if you have any questions.
Your sound card (if you want one) is a relatively simple decision. The SoundBlaster 16 is the defacto standard for sound cards, and is supported by virually all software. Some motherboards even include the SB16 chipset on them. If at all possible, buy your card in a jumpered version, rather than the SoundBlaster 16 Plug-and-Play that is popular today. Most vendors have jumpered versions available. There are also SB16-compatible cards out on the market, and they are definitely worth considering. Expect to pay around $80 for your sound card.
Possibly the choice that'll get me in the most trouble is the Ethernet card selection (if your system is going on a LAN). A Novell NE2000 clone is the cheapest choice you can make (the clones cost around $20), but most clones will hang the machine at boot time if the kernel is probing for other Ethernet card brands when the NE2000 is set to its default address of 300h. The solution is to either boot from a kernel with no network support (then recompile the kernel without the unneeded drivers), or to move the address of the NE2000 to another location. I've used 320h without problems to avoid this hang.
But the best way around the problem is to use a major-brand card. I currently rely on 3Com's EtherLink III series cards (the 3C5x9), which are universally supported, and software-configurable (from DOS, so keep a DOS floppy around). It's available in ISA or PCI versions, ISA being cheaper. This card costs around $90 from most vendors. I know that's more expensive than some motherboards, but it's a worthwhile investment.
If you are using dial-up access to the Internet instead (or just want a modem anyways), you can approach buying a modem with two alternatives. If your motherboard has built-in serial ports (almost all the non-VL bus boards do), then you could buy an external modem. I prefer them to internal modems, since the possibility of setting an address incorrectly is then gone, ad you can always tell if it is working from the status lights on the front of the modem. Internal modems generally cost a little less, but there's a greater risk of accidentally creating an address or interrupt conflict in the process of installing it. An additional problem is that many modems sold now are plug-and-play compatible. Unless you're already running Windows 95, P&P is a scourge on the Intel computing world (Macs do P&P in a fashion that actually works). Because most Intel-based OSs need to know the interrupt and memory location of peripherals at boot time, any inadverdent change caused by a P&P device can adversely impact the boot process. Linux can find many devices regardless (SCSI controllers, most Ethernet cards), but serial ports and sound devices are hard-mapped to interrupts at the OS level. So try to make sure that any such devices can be operated in a non-P&P mode, or in the case of modems, buy an external one if possible to avoid the situation entirely.
Remember, there are really two bottom-line reasons to build your Linux box yourself. One is to save money (and I hope I've shown you how to do that), but the real main reason is to have fun. Computing is a fun hobby, and building the system yourself can be a fun throwback to the early days when a computer was bought as a bag of parts and a schematic. I've been building machines like this for several years, and never had trouble--not to mention that I've gotten away with bringing in a lot of stuff under my wife's nose by buying them a part at a time! (Oops, the secret's out) So, for your next computer, give homebrewing a whirl. It'ss easier than you think, and what better companion for a free, home-brewed OS than a cheap, home-brewed PC?