GTX 570 is the faster card, hands-down. The 7850 uses less power,has more VRAM capacity,The GTX570 has it cons higher power usage, lower VRAM capacity,bottom line... IDK why so many people have said they are identical performance wise The 7870 is more or less identical to the GTX 570 in terms of performance but not the 7850. Not stock vs stock or max OC vs max OC.at least not according to these benchmarks http://www.techpowerup.com/reviews/AMD/HD_7850_HD_7870/26.html
You see different benchmarks will say different things test benches differ from system to another system.
And always performance gets improved with newer driver.Don't buy a 7850 wait for 660