I am olso going to build a ~500$ gaming pc. I have already chose it.So i think it may help you. I have done an amd based pc.it is olso 64 bit windows7 capable.
1.Processor: Amd phenom ii 555 x2 black edition ~100$
amd phenom ii 710 x3 ~95$
amd athlon ii 620 x4 ~87$
(phenom ii 555 x2 is best for gaming & athlon ii 620 x4 is best for multitasking)
2.motherboard: msi 785gm-e65 ~75$
asus m4a785td-m evo ~79$
ecs 785g ~75$
gigabyte ga-ma770t-ud3 ~82
(all are am3 capable)
3. Memory : Corsair/g.skill 2gb ~45$
(both are 1333 mhz if you looking for 1600(oc) mhz it will cost little extra but it will be good for oc )
4. Hard drive: seagate barracuda 250gb ~35$
5. Gpu: xfx 5750 hd ~145$
xfx 5770 hd ~160$
sapphire 5770 hd vaporx ~170$
(5770 will be better option for little extra)
6. Powersupply: you need 450 watt or more .there are many kind of product in market. take one which suits you best .
If you want to xfire in future them buy psu of >750watt.
7. Dvd drive: sony 24x ~20$ .
So the budget is ~ 500$