I love the 2560 x 1600 monitors; I have two, one for gaming, and the other for static stuff.
You have to realize that the graphics card will need to drive twice the number of pixels compared to the typical 1080P monitor.
I did OK with a 5870, but changed to a GTX580. It made a difference.
The 6970 or GTX570 will be reasonable, but if the extra $150 over a GTX570 or 6970 will not kill you, I suggest the GTX580.
The extra cost will hurt for a week or so, but the buyer's remorse and lingering desire for the best will last much longer.