Amazon.com: Valley Of The Sun 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,
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Upgrading the graphics in our 2011 iMac (first attempt) - YouTube
Amazon.com: Original Graphics Card GPU Replacement, for Apple iMac Late 2009 A1312 MC507LL/A 27-Inch Core i7 2.8 27-Inch Computer, AMD Radeon HD4850 HD 4850 GDDR3 512MB, MXM GPU Video Board Repair Parts :
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iMac Intel 27" EMC 2374, AMD Radeon HD 4850 Video Card Removal & Repair - iFixit Repair Guide
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