Setting up a big cluster requires some thinking to be done: where are you going to put the machines? Not under a table somewhere or in the middle of your office I hope! It's ok if you just want to do some small tests, but if you are planning to deploy a N node cluster you will have to make sure that the environment that will hold these machines is capable of doing so.
I'm talking about preparing one or more 19" racks to host the machines, configuring the appropriate network topology, either straight, single connected or even a 1 to 1 cross connected network between all your nodes. You will also need to make sure that there is enough power to support such a range of machines, that your air-conditioning system supports the load and that in case of power-failure your UPS can cleanly shut down all the required systems. You might want to invest in a KVM (Keyboard, Video, Mouse) Switch in order to facility access to the machines' consoles.
But even if you don't have the number of nodes that justifies such an investment, make sure that you can always easily access the different nodes, you never know when you have to replace a CPU fan or an hard-disk of a machine in trouble. If that means that you have to unload a stack of machines to reach the bottom one, hence shutting down your cluster, you are in trouble.