|The Unix Hardware Buyer HOWTO|
The PC Tech Guide offers pretty comprehensive descriptions of PC hardware technologies.
The Caveat Emptor guide has an especially good section on evaluating monitor specifications.
Anthony Olszewski's Assembling A PC is an excellent guide to the perplexed. Not Linux-specific.
Tom's Hardware Guide covers many hardware issues exhaustively. It is especially good about CPU chips and motherboards. Full of ads and slow-loading graphics, though.
The System Optimization Site has many links to other worthwhile sites for hardware buyers.
There's a Building Your Own PC page. It's more oriented towards building from parts than this one. Less technical depth in most areas, but better coverage of some including RAM, soundcards and motherboard installation. Features nifty and helpful graphics, one of the better graphics-intensive pages I've seen. However, the hardware-selection advice is out of date.
The Linux Hardware Database .
The Silent PC Reviews site has lots of good material on building quiet PCs.