If you want to play games at 2560x1600 then I would recommend a card based on what kind of games you are playing:
For Cs4, any mid-high range will do fine
-For games like Call of duty and normal non-intensive games then I think a GTX275 or GTX280 would be fine.
-However if you play really graphically intensive video-games and want them on high settings like Company of Heroes, crysis, bioshock etc then I recommend you get either a GTX285 or a GTX295 or you can buy two cheaper cards like 2x GTX260s or 2x GTX275s, because that resolution is really really high, its far higher then 1080p so you are going to need something "beefy and juicy" to power that.