After doing a couple searches, it looks like it's a no-go. (The PC arrives tomorrow, UltraNEO. I will still try it, though)
Looks like I will have to put my 3870 Mac and PC Edition in the PC and put the 2600 XT in the Mac Pro until the 5870 comes out. Can I just pop in the 2600 back in to the Mac Pro without doing any driver installation? The drivers are already in Mac OSX right?