iMac Intel 27" EMC 2429 Radeon HD 6770M Graphics Card
Apple opens graphics card replacement program for some mid-2011 iMacs - 9to5Mac
Amazon.com: Odyson - Video Card HD (6970M 2GB) Replacement for iMac 27" A1312 (Mid 2011) : Electronics
Amazon.com: 2GB Graphics Video Card Replacement for Apple iMac Mid-2011 27 Inch A1312 Core i7 3.4 MD063LL/A MD063 Desktop PC Computer, ATI Radeon HD 6970 GDDR5 2 GB, New GPU Upgrade Repair
Intel iMac 27" AMD Radeon 6970M 1GB Video Card, Mid 2011 DIY Parts replacement Video Cards