I don't think so. Mac graphic cards are soldered onto the board. I don't think there are any expansion slots on an iMac. iMacs have an ATI Radeon X1600 on a 16-lane bus interface and 128 MB
of GDDR3 video memory.
Your occupation says photography. Are you using something besides iPhoto? Aperture?
iMacs don't have expansion slots. If you want a mac that can be expanded and can make 3D animated movies, wait till the new Mac Pros come out next month. iMacs are designed for users and not a high-end pro user.
Anyway...what are you trying to do on your iMac?
[Edited 2006-06-26 08:47:17]
My name is Centrair but HND is closer. Let's Japanese Aviation!