Yeah, ATI cards are always gonna be a better buy. There is little reason to go Nvidia.
With your monitor, you're not going to need anything better than a 5850 for anything really. Even Crysis on very high/enthusiast settings will be more than playable on your 5850. The 5850 is slightly better than the gtx 285 (the card I have) and I play Crysis on highest at about 30 fps @ 1920x1200. I get like 35 fps on warhead. At your res, I would imagine the 5850 would give you 45-50 frames which is fantastic Crysis. Crysis isn't like other games. It will look good at that framerate. I don't know much about it, but there is something built into its engine.
However, a 5870 will up your frames even more. This really isn't going to help in anything, but Crysis though. Just remember, Crysis is like 6-8 hour game. The only reason to spend the extra $100 and get the 5870 is if you want to future proof some more. With your current monitor and the available games, there is no need for it. Just don't buy Nvidia. They are a waste of money.