+1 to the ATI Radeon HD 5770.
For the performance and future-proofing (DirectX 11 support), I would recommend this card. I needed a great budget gaming card and ended up with a 5770 which works great with almost every game, new and old. Can sustain 40+ fps on StarCraft II and even does a pretty great on on Crysis.
Go with the 5770, you won't regret it!