My 9800 gt handles crysis fine if only my CPU didn't bottle neck it, also sitck with the e7300 it's the same price as the e7200 with a faster stock Core clock, I will say the 9600 GSO is something I don't much hear of but the 4830 is a very good Bang For Buck AND I've heard 2 4830's in xFire are comparable if not faster than the $400 GTX 280 of course that'd be a different build altogether, but the 4850 is the way to go for single GPU BUT if Crysis is your thing then the 9800GT is a safe bet
I'm a nVidia guy two but there is speculation, of course not in Crysis but they do come close i quite a few benches, well maybe not at stock speeds but the factory OC'd and other OC'd editions really have the bang for buck factor anyways I'm not here to hijack the thread
4830 is very comparable to a 9800gt in most games and their both great but the 4830 is normally a bit cheaper. And a 4850 is a good bit faster in most games.
I'm not sure which is the best of each but here are some cheaper ones