Depends on what you're playing.
Realistically, expect the desktop GTX 570 to push as many as 50% more frames for new DX11 games that are heavy on shader effects, at high resolutions (1920x1080 or 2560x1440) assuming an equivalent CPU. This isn't surprising considering the typical power consumption increase between idle/loaded for a 570-equipped machine will match a loaded 27" iMac at 60% brightness that's loaded (i.e. if a 570-equipped gaming computer uses 150W at idle and 330W loaded [excluding display], that's on par with the 27" iMac's power consumption when loaded)
When you have that much more of a thermal envelope to play with, you can do a lot more.