Nvidia has better drivers, it has a dual slot cooler, it can overclock much more, and in most of the reviews I've looked through @ 1280x1024 it trades blows w/the 4850. The 4850 only really beats it playing Race Driver GRID at high res with AA.
But on another note, I bought an eVGA 9600GT SSC (super super clocked) for $65 w/rebate and at 1280x1024 it was maxing out everything (CoD4, GRID, etc.) but Crysis. The only reason I sold it was to turn a profit and wait for a better deal.