So you need a case, psu, mobo, cpu and a gfx card then?
Well i would say get the Intel core 2 extreme x6800 (Previously named conroe) as they are the fastest cpu around at the moment that absolutely obliterate the Amd 64x2's but...that would eat all of you budget.
So Id say: AMD Athlon 64 X2 4400+ $250.00
DFI LANPARTY 939, mobo $190
SeaSonic S12-600 ATX12V 600W $160
7900GT 256MB $260 (Estimate)
Case isn't something i can choose.
and those prices are from newegg so if you shop around you can probably find them cheaper.
Thats a quick list, so id suggest posting this same question over at
www.techimo.com they have good advice and good people that will help you much better than i can.