I suggest you get two GTXs if you plan on gaming at 2560x1600, that's really the only graphics cards setup that can push that kind of resolution at a somewhat acceptable frame rate.
Emp, you are spoiled!!!
I was gaming in 2560x1600 with a pair of 7600GT SLI. It was ok. Nothing maxed but still looked amazing and ok FPSs.
I just upgraded to a single GTX and man, it is a lot more than "somewhat acceptable".
But I have to admit, there are some really poorly writen games out there that make me have to turn the res or eye candy so I can decent FPSs.