If you manage to get PC cards working in OS X I may just have to betroth my (unborn, in fact unconceived as of yet[EDIT]but I'm working on it with as many people as I can[/EDIT]) daughter to you and worship you as a god of OS X and the saviour of Apple for the rest of my life.
After a bit more research, this might be easier than I thought it would be, yet harder at the same time.
I'd write a "fake" UGA driver that is loaded by EFI at boot time. This "fake" UGA driver would report that a video card was present, clock rate, frame buffer, address space, etc, etc.
This would make EFI think there was a valid video card present and proceed. The thing is, there *is* a valid video card present, it just doesn't have UGA firmware of its own. The information that my fake driver reports would be correct, so when the NVIDIA or ATI kexts load up, they'll take care of everything else.
Now, the big issue with this is as follows:
I'd need to write a different fake UGA driver for
every variation of PC graphics card. By that I mean, an eVGA brand 8800GT would require a different UGA driver than, say, a PNY brand 8800GT.
Is there an easier way to do this? Yes, I think there may be.
People in the Hackintosh community have something called Titan.kext.
What Titan does is manually set all the video card information in the IOReg plane at boot time, at the operating system level, rather than the EFI level.
The problem with just outright using something like Titan is that your Mac Pro is not going to boot if you
just have a PC video card installed.
EFI won't be able to initialize a video device and thus will refuse to boot.
There's two ways around this:
1) Create a single fake UGA driver that tells EFI there's a valid video card in the system, thus allowing the system to boot. (You won't have any graphics on screen from power up until the OS loads with this method.)
2) Keep a 7300GT or HD 2600 installed in the system so it has a valid video device on boot.
Either way, once the system gets into the OS loading stage and Titan is loaded, we'd dump the appropriate values for your video card right into the IOReg and boom, video fires up off your shiny PC 8800GT (or whatever other compatible PC card you have).
Now, I don't think Titan has been updated to work with Leopard, but his website has all the basic information available to make my own version, specifically for Mac Pro users that works with 10.5.
This sounds complicated, but once it gets programed and tested, it should be really simple for an end user to install. The best part is, it
won't require any flashing!