opteron 144 - $130
ASrock Dual Sata - $70
one of these cases
depending on which you like(both are good and cheap) - $50
and an enermax 535w - $90
and a 7600GT - $200
total = $540 + shipping
the 7600 is nice but if you want to save money a 6800GT is also decent for now. the 6800s are quickly becoming low end though, but hey..if it works for you, go for it.
P.S. don't need RAM or hdds or OS?