What SSL said, Best value would be either the GTX480, GTX660 or GTX660Ti, The Ti is about 50$+ than the 660. The GTX480 is a bit harder to find on most websites, But if you can get your hands on one, You should go for it, It's slightly cheaper than the 660, And it delivers pretty much the same performance, If not better.
Get one of those three, And you're in business.