[Mark Petereit] “We have an old G4 Mac Pro sitting in a closet.”
No, you don’t. You have a Power Mac G4. The Mac Pro has Intel CPUs, whereas Power Mac G4 and G5 models have PowerPC CPUs. FCP 7 won’t run on non-Intel machines. Yes, some people have hacked it to run on the G5, but it won’t run as well or support ProRes capture. As for the G4, just forget it.
[Mark Petereit] “What if bought another old G4 Mac Pro and added BlackMagic capture and GigE cards to each?”
The Power Mac G4 only has PCI slots. Even the oldest generation of Blackmagic capture hardware requires at least PCI-X slots, which weren’t introduced until the Power Mac G5 was released. The latest generation of capture cards use PCI Express, which was only featured on the very last model of Power Mac G5. All Mac Pros have PCI Express slots.
You’d be much better off saving your money for a used Mac Pro – the first models can be found for a little over $1000 these days, and they are infinitely more capable and expandable than the Power Mac G4 or G5.