I checked the
Apple Store PowerMac configuration page, and they do have NVidia GeForce 7700 and 6600 as options!
I also checked ATI, and they have Radeons for the Mac too (
http://shop.ati.com/searchresults.asp?dept_id=31)!
And if the rumor that PowerMacs will have the new Intel chips (Conroe, Merom and Woodcrest), these rigs will be one mean gaming machine!!!
