amd 5770 vga card died, need compatible replacement (2011 27" EMC)

Apple offered three AMD boards for this series:

• 661-5967 - Radeon HD 6770 (512MB)
• 661-5968 - Radeon HD 6970 (1GB)
• 661-5968 - Radeon HD 6970 (2GB)

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.