For anyone looking for something like this...its a roundup of pretty much all the graphics cards that have come out this summer. It includes the two highest end cards currently out from both companies...the 4870 from ATI and GTX 280 from Nvidia. If you want a basic summary, GTX 280 beats the 4870 out in most cases. It seems the GTX 280 is better with higher resolutions, while the 4870 handles AA extremely well. In a lower res high AA benchmark the 4870 actually beat out the GTX 280. However, as they upped the res (without AA) the ATI cards seemed to crumble compared to the Nvidia cards. Keep in mind though, were talking $300 for the 4870 vs currently $500 for the GTX 280.
http://www.bit-tech.net/hardware/2008/07/11/summer-2008-graphics-performance-roundup/1