so... hold on a second... I could just plug in the mac 8800 along with my 7300, right?
That way my machine can boot using the 7300 EFI and I could use the 8800 in OS X, right?
When using windows via bootcamp, I could just disable the 7300 all together... please correct me if my thinking is wrong. I am tiered of waiting for another excuse.
(I will never buy another mac pro after this debacle)
After scouring the web looking for anecdotal evidence that this sort of thing will work, I will save you the trouble and give you my findings here.
You can run pretty much any card in bootcamp but its a completely different story when you want to boot back into OSX. I dont think you can ever get a non apple EFI card to work with OSX just by plugging it into a slot and the best you can hope for is that it doesnt get in the way of your normal card. There are apparently some people who actually take their card out when they boot into OSX as they mostly use bootcamp but its not something I want to do.
If your card is Ok while you are running OSX, you can unplug your video cable from your new card back into your apple card...its better than removing the card but I still dont want to crawl under my desk and do this everytime i use OSX.
The hackintosh people have these cards working with OSX but its not simply plug and play. They have written third party drivers and they apparently need to tinker a bit with the bios or EFI.
One person here did manage to get the ATI 3870x2 to work in his Mac Pro but its not clear if this was with all of the hackintosh tinkering or if he simply plugged it in. Also, I was under the impression he had a 2600 card in there as well and was unplugging his video cable and replugging it back into the 2600 when running OSX..but I am not sure about this. He did mention the 3870 fans went nuts when he was running OSX for some reason. It does seem that ATI cards might be more likely to work than Nvidea cards as seem to be ATI drivers included in 10.5.2.
Also, remember that you need to separately purchase the ATI 1900 power cable as there is no place in the Mac motherboard for the 6 pin molar to 4 pin cable that comes with the card. Do not try to run this off your superdrive cable as someone did try this and it did not work. Unfortunately, this cable is only available from ATI and you will not find it/or a converter easily. I have actually been looking for awhile without success.
So in other words, the only hope is to try and run an ATI card and buy the cable. Search this board for 3870 and you will see that one poster did seem to get this working but its unclear how much he had to do to get this done.