Nvidia GTX 680. Well, to be honest, an Nvidia 8800GT, but I've ordered a GTX 680.
Not to rain on your Parade, but does your board support PCIE 2.0? If not your still running the 1.0 pipelines, which limits your new card to half of it's capable performance..
Hehe. I had an Intel Core i7 system until the motherboard broke last month. Why I still have a GT 8800 is because I'm to greedy to update it, and I didn't need it as I was mainly doing rendering and casual gaming. After I finally got my money back for the motherboard, I decided to upgrade to newest Intel i7 generation - with PCI-E 3.0 - and why not get a new GPU too?
Edit: Bought Gainward Phantom GTX 670, instead. More bang than a standard GTX 680, saving me equal to USD 150 in Norway.