When I finally get around to installing Leopard, I'm going to have to make a tough decision about whether I want USB/FW or 10/100 ethernet, as I only have one expansion slot left to play with. I'm inclined to think internet sharing with another computer over FW would be the way to go.
I have no idea whether this works with any PPC Mac as those are VIA chips onboard but things like this do exist surprisingly enough
http://inlandproduct.com/pciusb202port2port1394afirewirerj-45ethernetcombocard.aspx
Maybe someone has had experience of something a bit cheaper than Sonnet/Macally that also is Mac compatible.