i am using the 4870, no efi or kext needed anymore thanks to pc efi boot file from netkas (netkas.org, you will learn a lot).

boot with the -v flag for verbose mode (in your /extra/com.apple.boot.plist or at the boot prompt) so that you can see where you are getting stuck and post the result here.

prior to the built-in fix for the 48x0 cards from netkas i was using the evoenabler.kext which was working well. I think you can find that on netkas' site as well.