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 :
21.5 iMac Mid 2011 12,1 A1311 820-3126 Logic Board Core I5 "CPU Not In - Part Haven
HOW TO: Upgrade The CPU from an Intel Core i5 to an Intel Core i7 on a Mid-2011 21.5inch iMac - YouTube