1. Use Freedos or Windows and (respectively) atiflash or atiwinflash to dump the ORIGINAL firmware from the card. Save it for later. Restart to OS X.
2. Get
http://forum.netkas.org/index.php/topic,5619.msg19874.html#msg19874 the file on the bottom (
MAC7970.ROM)
3. Get
https://cloud.mail.ru/public/8c978735fc99/bundle.tar.bz2 and double-click to uncompress to a folder.
4. Put the original firmware from (1.) into this folder.
5. Click on Apple → About this Mac → System profiler → Graphics/Displays. Look for "Vendor" and "Device" in the right pane. Note down the DEVICE number (will be something like 0x6789). Actually 0x is redunant, you need 4 last digits.
6. Open terminal and navigate to your folder (use cd /Users/... etc)
7. In terminal, issue the following command, assuming:
a) your original firmware has been saved with name "cardname.rom"
b) your Device ID is 6789 (it WILL not be 6789 BTW)
Code:
./makerom.sh --efifile=7950mac.efi --romfile=efiromheader.rom --originalrom=cardname.rom --devid=6789
8. If there are no Java and Python errors in the output, you will have a new proper EFI firmware file sized 128k. However if there ARE any errors, this file is useless, remove it. Reinstall Java from java.com, restart the computer, check for any updates in App Store. Go to 6.
9. Save the new firmware to an USB stick and back to Windows (strongly recommended).
10. Use WinAtiFlash to upload the new firmware to the card. Once done, shut down and reboot to Windows.
11. If everything worked OK, you can see the Apple logo on boot.
12. If the above doesn't work and you have the original firmware from the card dumped, just send it over to me, I will do the terminal stuff.
References:
https://forums.macrumors.com/thread...-boot-screens-pci-system-information.1808938/
http://forum.netkas.org/index.php/topic,5619.0.html