Upgrading GPU Graphics Card on iMac 2011 - YouTube
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Amazon.com: Sunvalley 2GB Graphics Video Card Replacement for Apple iMac Mid-2011 27 Inch A1312 Core i7 3.4 MD063LL/A MD063 Desktop PC Computer, ATI Radeon HD 6970 GDDR5 2 GB, New GPU Upgrade : Electronics
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How to Upgrade/Replace GPU Graphics Card on Mid-2011 21.5" iMac - YouTube