Well, I don't agree again with the graphic card, choose something like GTX275, GTS 250 it's nothing really new, is a rebadged 9800 GTX+ with some chances.
If you are not going to use SLI, a 600W PSU is enough, like this CORSAIR HX-620W MODULAR, for example.
A possible choice:
CPU: AMD PHENOM II X4 955
Motherboard: GIGABYTE GA-MA790XT-UD4P
RAM: GSKILL KIT 4GB DDR3 1333MHZ NQ
Graphic Card: Nvidia GTX275 (BFG, EVGA, XFX)
PSU: Corsair HX-620W
With this system you are not able to use SLI, this only supports Crossfire (dua ATI cards).