Upgrade 24” iMac video card - ATI Radeon 256MB to NVIDIA GeForce 512MB - Ask Different
Amazon.com: Genuine Graphics Card GPU Replacement, for iMac 2008 24 Inch Core 2 Duo 2.8 Desktop PC MB325LL/A A1225, ATI Radeon HD 2600 HD2600 Pro GDDR3 256MB MXM Video Board 109-B97157-00 : Electronics
Original Ati HD 6970M 5750m 4850 4670 Graphics Card Video Card 1GB 2GB 109-C29657-10 For Apple iMac 27" A1312 GPU 2009 2010 2011 - AliExpress
2010 iMac GPU Upgrade to NVIDIA GTX 765M - YouTube