Again, RAM is the biggest key to loading up more of -anything-. While you can load 100 clients on 256mb of ram, you're not going to be moving anywhere. If you can, 3gb on normal XP or 4gb on x64. If possible, go 6-8GB on x64. My next system I'm building here in a month or two will have 8gb. I don't know what to expect with it as I've never used a system with more than 4gb but what I do know is that 2->4gb was a vast increase in power. When I first built this comp I was using my old processor before I got my x64 so I saw the power difference of going from 2->4gb of ram and a processor switch of 2->2.4x2. The ram helped far more than the processor did.
Minimum of at LEAST 2gb of ram. 4gb is recommended. As for processor, if you're looking for dual/quad core, just get whatever looks good really. As far as Intel goes, most of their recent processors are decent. Just stay away from PentiumD :P If you go AMD, hold off for the quad cores they're coming out with. Hopefully it won't be a disappointment.