iMac Intel 27" EMC 2374, AMD Radeon HD 4850 Video Card Removal & Repair - iFixit Repair Guide
APPLE IMAC 2011 A1312 AMD HD 6970M - Graphics Card with NEW 6970M GPU & 1 GB RAM £399.99 - PicClick UK
Amazon.com: Original Graphics Video Card GPU Upgrade Replacement for Apple iMac 21.5 Inch Late 2011 Core i3 3.1 All-in-One Desktop PC MC978LL/A, AMD ATI Mobility Radeon HD 6750M, MXM VGA Board Repair
Apple Initiates Video Card Replacement Program for Radeon HD 6970M Equipped Mid-2011 27” iMacs