According to the techPowerUP review, the 5870 beats the GTX 275 in 4 out of 19 games (CoJ, Stalker, Stalker: Clear Sky, Quake 4). Overall, the 295 is 15% faster at 1920x1200.
http://www.techpowerup.com/reviews/ATI/Radeon_HD_5870/30.html
According to the PCGH review, the 5870 beats the GTX...