Amazon.com: Valley Of The Sun New 2GB Graphics Video Card Upgrade for Apple iMac Mid-2011 27 Inch Core i5 MC814LL/A MC814 A1312 Desktop All-in-One PC, AMD Mobility Radeon HD 6970M Replacement :
GPU Video Heatsink 730-0630-A with Sensor Original iMac 27 inch A1312 2011 for Radeon 6770 6670 6750 - used original spareparts for Macs
iMac 27' Mid 2011 GPU Issue | MacRumors Forums
Cracking Open the 27" Apple iMac (2011) | TechRepublic
Apple iMac 27" Mid 2011 Video Card - ATI Radeon 6970 with 2 GB GDDR5 m - Part Haven
How to Upgrade/Replace GPU Graphics Card on Mid-2011 21.5" iMac - YouTube