I think Jarod hit on something, alot of people think 1 GB of memory is enough, when you think about it, alot of games require 512mb, and if you are a power user then 1gb of ram can be a real bottleneck! I would say get the following
AMD 3500, with an ASUS board (i'm an asus fan)
and couple that with 2GB of ram and at least a 6600GT nvidia card, that card will play games and with that processor and ram amount you would be hard pressed to over work it. Plus unlike the older prescotts, which were better known as the PressHott. because of how hot they got, the AMD 64 bit chips have the feature where it doesn't use all of the CPU power when its now being used, in order to cool it down, its called Cool -n- Quite. YOu could probably build that for around I'd say 550$, and if you need a case a simple 40$ one will do fine as long as it has room for 2 fans
Edit: I say get the asus because I have one and have had it for a while now, and it has NEVER even hickuped!

I would go with the 939 socket also, dont get the sempron or socket 754 ANYTHING (espec. the amd 64 3400 @ tigerdirect) they didn't have it until they cut the prices of the 3500 by 22%, and now its just a rip off because the chip has been discontinued, so dont get that as you would be shooting yourself in the foot (or stepping on your ding ding) by updating to something that isn't really being supported anymore
