By the numbers: Apple's new iMac vs. the Windows 8 all-in-one competition - The Verge
21.5" Late 2012 iMac | Display or GPU issue - Ask Different
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 :
imac 27 inch 2013 graphic card faulty repair - YouTube
2010 iMac GPU Upgrade to NVIDIA GTX 765M - YouTube