This may come as an interesting recommendation. To futureproof your particular computer I would buy a 780i.
You already have an 8800 ULTRA and once the price comes down you can purchase a second 8800 ultra and increase your fps quite a bit.
I will probably get flamed for recommending an Nvidia chipset but at stock they work quite well, overclocking they are a bit on the flaky side though, but you won't be overclocking.