You have a 4870 that doesn't have the Apple EFI? If it is a reference model card, it can be pretty easily flashed with Mac EFI (Apple boot screen). I have flashed these for 4 of my friends with good success.
I know from personal experience that installing those booster cables (if not present) is the biggest pain in the butt part of this job. Other than that it is super easy and anyone can do it.