I take it you have a Core i5-650 or Core i5-655K? If so your CPU is probably just fine for gaming.
Start here, and get the most powerful GPU in your price-range: »AMD Radeon HD 7750 7770 7850 Prices Slashed
You're probably looking at a GTX 660 Ti or a Radeon 7950. I'd go for the 7950.--
If we lose this freedom of ours, history will record with the greatest astonishment, those who had the most to lose, did the least to prevent its happening.