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 2009 to 2011 (A1311/A1312) Upgrade Guide – MacintoshMen