MBP 2.6 with Nividia vs iMac 2.8 with ATI performance

I do a lot of image and graphics work on my XP windows machine....my wife has been using a G4 iMac for years, just browsing and email....she loves her iMac...I do to, since I don't have to play sys admin on it.....I will purchase a new machine for myself....in doing my research, I don't worry about HD size, since I use external drives to archive my work...portability would be nice but performance is key since my workflow needs are very demanding....which machine has the advantage in the graphics department? the forums help is very appreciated.