I just build a gaming PC for about £800 (roughly $1200)
AMD Phenom II X6 1090t With Corsair Hydro H50 water cooler (so I can OC to 4Ghz)
Asus M4N75TD Mobo
Gainward GTX460 1GB Golden Sample GPU
Komputerbay 8GB 1333Mhz DDR3 RAM
Corsair CX600 600W PSU
Samsung F3 1TB HDD
Nzxt Hades case
Win 7 (64 bit)
I'm very happy with this spec.
You may want to consider spending slightly more on a better asus Mobo with USB3
If you don't like the AMD AM3 range (very good value), I would recommend the classic i5 760 quad core (socket 1156, so you will need a different mobo) or an i7 860 which is a more expensive quad core, also on socket 1156.