One thing we still need to know is budget. We can suggest a bunch of things, but unless we know how much you want to spend, we may or may not help you at all.
For $150 you can get a Radeon HD 5770, which would be a little under-powered compared to your build.
For ~$300ish you can get a 5850, which is a great price/performance value and a great card.
For ~$400ish you can get a 5870, which is the best single GPU card out there and very powerful, but not as good price/performance value as the 5850.
For ~$700ish (when you can find them) you can get a 5970 which is the best single card out there right now (dual GPU) and is amazing, though pretty pricey.
If you want a little more bang for your buck right now, and are willing to crossfire, you could go for 2x 5770 for $300 as well, which will perform a little over a 5870.