amd 5770 vga card died, need compatible replacement (2011 27" EMC)
What other vga card will fit the iMac mounting board and the heat sink?
Apple offered three AMD boards for this series:
Be aware these boards have custom firmware so unless you have the tools to reburn the EEROM you need to stick with the Apple boards.
Apple also changed things with newer macOS releases so unless they support Metal services you can’t run these older GPU’s. Some people have used NVIDEA boards which offer Metal services, but they often need EEROM programing and the NVIDEA driver is running in compatibility mode so many hardware improvements NVIDEA added are not supported. Apple never gave NVIDEA the needed info to update the OS drivers.
It’s also likely macOS will be dropping Intel support soon, if not more limited given the new features won’t be made available on Intel systems.