Are you taking about the iMac? I went from a 24" 3.06 GHz with the 8800 GS to a 27" 3.06 GHz with the 4850. Playing at native resolutions in bootcamp, the 27" is equal to or slightly faster than the 24". I don't have exact numbers, but Team Fortress 2 is more consistent about its framerate now (fewer frame drops). Crysis is slightly faster at 1280 x 800 all-high settings than the 24" was. Very high settings are still too jittery at 1280 x 800.
Don't ask me how it's possible to get higher frame rates with 62% more pixels and a video card that is only 30% faster. I was pleasantly surprised and don't understand it myself.