My Airport card arrived. Installed it in the G4, which was tougher than I expected. Getting the machine open was easy, though I had to unplug a couple of the cables. It also didn’t take me long to work out that the piece of green PCB attached to the Airport card had to be some kind of extra part for use in other computers (iBooks maybe?) and not something I had to use.
The bit that took the time, though, was working out why the card wouldn’t go in the slot. It turned out that the manual showed it being inserted the wrong way up, and there’s a pin which is invisible from the angle I was looking, which prevents you from doing that.
Anyway, I now have IEEE 802.11b support. Tomorrow evening I’ll go to Micro Center and get a wireless router and firewall.