I have the imac 27inch i7 with 8gb of ram from last November, it runs great but I always run into this problem with photoshop. Attached is the screenshot.
EDIT;
After searching google I found http://forums.dpreview.com/forums/read.asp?forum=1017&message=29776384 and this may have fixed it for me.
EDIT;
After searching google I found http://forums.dpreview.com/forums/read.asp?forum=1017&message=29776384 and this may have fixed it for me.