Amazon.com: 2GB Graphics Card GPU Upgrade Replacement, for Apple iMac 27" 2011 Desktop Computer A1312 Core i7 3.4 MD063LL/A MD063, AMD Radeon HD 6970M GDDR5 2 GBS, MXM VGA Video Board Repair
Amazon.com: Original 2GB Graphics Video Card Upgrade Replacement for Apple iMac Mid-2011 27 Inch A1312 Core i7 3.4 MD063LL/A MD063 Desktop PC Computer, AMD ATI Radeon HD 6970 Spare Parts Full Tested :
iMac Holy Grail achieved ! Boot screens on 780m in 2011 iMac 27".
iMac Intel 27" EMC 2374, AMD Radeon HD 4850 Video Card Removal & Repair - iFixit Repair Guide