well your resolution is one of those which i recommend SLI or CROSSFIRE, but not 2x midrange cards , for example 2x8600GT wont perform very well in SLI , if u can get 2x8800(GTX-GTS 640) then go for it , if not , go for a SINGLE 8800GTX, dont go for ULTRA, its overpriced also in fact 8800ULTRA = 8800GTX OC'd , u can OC a 8800GTX to make it ULTRA or even further also the difference between 8800GTX and ULTRA isnt noticeable
Bad news: for that resolution you will need two 8800 GTX cards. Forget mid-range cards. You could get one now and add the other one later if you're not happy with the performance. What kind of motherboard and power supply do you have? You will need something based on 680i, like P5N32-E SLI, for best results. A PSU like Toughpower 850W would do nicely.
Experiment with these benchmarks to get an idea. I didn't find benchmarks for 2560x1600, but 1920x1200 is useful. You will get a bit more than half the fps shown in those benchmarks, at your resolution. For example if you see 58 fps at 1920x1200 it means you'll get about 30 or 32 fps at 2560x1600 in the same game with the same card (or pair of cards).