Run it and tell me what you think. :thumbsup:
Trust me my GTX570 slaps around my 6950 in GTA4, the difference is night and day. It never ran properly for me with a 6870 either.
That was just an example of a couple of games, there are more. Like opengl source ports, mods like Morrowind's MGE seem to perform poorly with ATI cards, FSX, Lost Planet 2, etc. My point is at with the GTX580 it is at worst on par with a 6970 and those are the games that everyone likes to mention even if they artifact like crysis.