For a long time I wanted to use a G4 Mini as the back up server in my house. I spent a bunch of time looking at schemes for putting gigabit ethernet in the thing. Nothing was going to be particularly good. The most promising avenue of research was to reverse engineer the pin out of Apple's Mini-PCI slot by comparing broadcom wifi cards for the mini to PCI wifi cards with the same chipset, then create a card with a Realtek 8169 on board using the pinout I would have discovered.
I still hadn't figured out how to bring the ethernet cable out of the machine when I bought another Mini (to experiment on) on Ebay, and it turned out to be a 2010 C2D Mini instead of a G4 (the description was kind of vague). At that point, I abandoned the whole scheme.