Chosen Solution
I got an iMac late 2009 (i7, HD4850 512Mb) runnin latest macOS and my graphic card just died. I can find cheaper HD6970 than the exact replacement for my card. Has anyone done the upgrade recently? Will it work with High Sierra?
Apple has custom firmware on their graphics cards so you just can’t throw in any card. So unless it has an Apple logo’ed label on the firmware chip you are taking risks. Here are the two cards that came with your system: ATI Radeon HD 4670 with 256 GB onboard RAM - Apple P/N 661-5314ATI Radeon HD 4850 with 512 GB onboard RAM - Apple P/N 661-5315 I would focus on one of these using the P/N to locate. Its possible to use a different board but you have to be careful! You need to use one that has the same chip orientation so the heatsink covers it fully and you can’t use something that runs hotter as the heat sink you currently have needs to work. Don’t forget you’ll need to clean & apply new thermal paste and pads! Arctic Silver ArctiCleanArctic Silver Thermal Paste100mmx100mmx0.5mm Conductive Silicone Thermal Pad100mmx100mmx1.5mm Conductive Silicone Thermal Pad100mmx100mmx1.5mm Conductive Silicone Thermal Pad
