iMac Exchange repair programs
Two iMac Intel exchange programs may be available through your local authorized service technician:
iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program
http://www.apple.com/support/imac-harddrive/
Note, if the link says they are still available and you have difficulty getting a repair, contact AppleCare or customer relations.
This is just a user to user forum. This tip is for your info only, and indicates no guarantee of service.