SOLVED: What GPU's are compatible with iMac 27" 2011? - iMac Intel 27" EMC 2429 - iFixit
Amazon.com: Genuine Graphics Video Card for Apple iMac Late 2009 A1311 Core 2 Duo 3.06 21.5 Inch Desktop PC MB950LL/A MB950, AMD ATI Radeon HD 4670 GDDR3 256MB, MXM VGA Board GPU
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