Tutorial to replace the Video Card on an Apple iMac 27" A1312 and Similar Models.
The tools you need are the following
1. Two GPS Car Windshield Suction Mount to pull the front glass
2. Star-Shaped Screw Driver T10
3. Star-Shaped Scre Driver T8
4. Optional Powered Screw Driver
You can buy the replacement video cards from eBay.