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 :
Upgrade iMac Intel Core i3 CPU to Core i7 - iFixit Repair Guide
iMac 2009 to 2011 (A1311/A1312) Upgrade Guide – MacintoshMen