To some degree, I've recently come away with the notion that you don't get rid of the existing BIOS, but rather add an EFI partition to it that the Mac would recognize. I've seen numerous guides on the internet about flashing the BIOS on the video cards and also ones that speak about adding the EFI BIOS, but in the haphazard forum approach, it isn't clear how this all works.
The card is supported on the Mac once it boots, but I would like to get boot screen back since my old 8800GT has been baked in the oven a few too many times by now :-)
I've got a PC video card that I'd like to get boot screen support for on my Mac Pro.
