I played Witcher 2 on my old PC with an 8800GTX512. I jumped between 15-30FPS throughout (cept for certain places in Vergen, which grided my poor old GPU to dust).
The best settings for a middling PC is high character and landscape texture resolution for cards over 512MB vram, lowish effects. As in ingame DOF enabled (it barely effects performance at all), no cinematic DOF, no AO, no wet surface effects, no motion blur (shame, cuz I love motion blur), and NO UBERSAMPLING, as that's the one setting that will destroy your framerate on everything but the highest of the high end gaming PCs. You could probably play the game 2560x1440 on a high end iMac and still maintain 30 FPS this way. It still looks stunningly good, and you only sacrifice some of the atmospheric effects.
This applies to the Windows rev, though the same will likely apply to the OSX version.