PC Cards will work to a certain extent in a MacPro - ie under windows once it's booted accorind to this article:
http://www.anandtech.com/mac/showdoc.aspx?i=2832&p=7

As the MacPro has configurable PCI-E lanes you could configure the lanes to run at different speeds, theoretically you could put your big fast graphics card in the 16x slot for use in XP/vista-DX10 and put the mac compatible in one of the other slots at a lower speed - this would cripple it slightly for use in OS-X but it is doable.
See this article:
http://www.barefeats.com/quad14.html
and this infodoc:
http://docs.info.apple.com/article.html?artnum=304122

you would still have to swap the monitor between cards on the reboot.

It is most likely doable - not sure if its worth the effort  though.

http://www.barefeats.com/quad14.html

I'm surprised there is no way to use the Linux drivers for the 8800 on mac. But then mac is based on BSD and BSD is kinda useless.... It might be a possibility with a lot of hacking though

A 1900xt is a really fast card.  I have several and would see no reason to upgrade unless you wanted to play dx10 games on your Mac.  That wont happen though as MPros are not used for gaming anyway.  If you want games, get a PC or a console.  Yeah, Steve said games were coming back to Mac, but I still dont see it ever becoming a gaming machine....and with that there are no hi end gaming cards.

Skipping ahead... I think this is a good point, and more evidence that there needs to be some sort of mini-tower to fill the headless computer gap between the Mini and the Pro. The Pro was never intended as a gaming machine, and really, the high-latency RAM it uses doesn't help with gaming anyway. Giving the Pro a bevy of GPU options that are best for gaming doesn't do a whole lot of good, and those options probably would not sell as well as, say, cards better suited for multiple purposes, or workstation GPUs.

Now if they had something smaller in size than the Pro, with a full-sized 16x PCIe slot, 4 RAM slots, a couple HDD bays, an optical drive, and a couple more slower PCIe slots... I'd buy one in a heartbeat. Granted, it wouldn't be Xeon-equipped. More like high-end Core 2 Duo's. Ah, but one can dream.

The Strangedogs people, if anyone didn't know, have moved. The old forum is shut down and they're now at The Mac Elite.

Maestro is 100% correct, by the way: Without existing Mac ROM data to go off of, you can't just magically make a ROM to flash into a PC card. You could, most likely, run an 8800 in a Mac Pro via Boot Camp, but under OS X, you'd get nothing.

Just an update here.  With the new intel cpus coming next week, we should see a new revision of the Pro in the next month or so.  With that in mind, Apple may be working on getting some of the 8 series Nvidias working with it.  We may see the Gtx, but most likely the Gts and definitely not the Ultra.  Still dont see dual graphics coming just yet, even with Apple getting back into the games market.