Answer to: AMD Radeon HD 6750M 512 MB hardware upgradeHi, I have a similar problem. But worst! My card stoped working. I need to get another one to get my iMac back working again. Presently only works with Team Viewer screen sharing app, or maybe with an extra screen, which I didn't try yet. Where's the best place to buy an used trustable one? There's a lot of AMD Radeon HD 6770M and 6750 M in the net, But, I'm afraid they may not fit or work in my mid 2011 iMac 2.5 GHz intel EMC 2428 A1311 Thank you.