iMac Intel 27" EMC 2429 Radeon HD 6770M Graphics Card
Amazon.com: Valley Of The Sun 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 Repair Parts :
Amazon.com: Genuine Graphics Video Card GPU Replacement for Apple iMac Core 2 Duo 2.0 20-Inch A1224 MA876LL Desktop Computer, ATI Mobility Radeon HD 2400 XT GDDR3 128MB, MXM GPU VGA Board Repair
iMac Intel 27" EMC 2374, AMD Radeon HD 4850 Video Card Removal & Repair - iFixit Repair Guide