The Corsair CX600 should be perfect for that build.
Like the others said, the HD 6870 is the way you should go.
From a GTS 250, a GTX550Ti, would be little to no improvement, an HD 7750, maybe a minor 5-10% upgrade, a HD 6850, should see a significant boost, and a HD 6870, even better.
And buy more RAM. If I'm not mistaken, you'be got an extra slot for adding another 2 GB stick, so do that.