What parts do you already have and what will the system be used for? I would suggest getting 4 GB of 1066MHz ram, everything will perform better.
I have a 420w ps and ah... a case. I'm planning on a mb that has video/audio on board. The hd is EIDE, so unless I get a mb that has EIDE support, I'll probably need a EIDE hard drive. So I'll need mb, cpu, ram, and a hd.