Can't you do it with a GTX 670 or GTX 680 in your Mac Pro and put a TripleHead2Go on one of the outputs? That would give you a total of 7 outputs.
Anyway if you want to go to the PC route, just get a Blackmagic Intensity PCI card and use that to capture the video from the Mac Pro. As far as a good video card, the Resolume developers use a Radeon 7970 in their PC build.
You could also just run video off both computers, you can control the other copy of Resolume via midi over Ethernet, you should be able to sync a Mac and a PC running the midi over Ethernet, you need to install the RTPMidi driver on the PC and the driver is compatible with the network-MIDI included in every Mac.