Hello I had a problem with my graphic card nvidia geforce 8800 gs on an imac 24' 2008, like a lot of people. I removed the graphic card, put it in an oven at 195° C for 8 mn and it works again now.
I arlready did the same operation with a mbp 2007 where the chip is soldered on the mother board but then it lasted from 3 to 6 months before I had to do it again, 5 times now in 3 years. I hope that it will last more with the imac, but still it's a cheap way to solve the problem.
On this site you have the long story and the procedure :
http://russell.heistuman.com/2010/04/27/cooking-the-books-or-baking-my-macbook-pro-logic-board/
And on ifixit you'll find the guide to remote the graphic card :
[guide|13765]
Good luck