Orinoco in Airport slot of G4
Too cheap to spend your junk food budget on an out-of-production Airport card?
Use an Orinoco card. Pictured here is an orinoco silver, obtained off ebay for USD10, flashed to gold via alchemy, shed of it's metal cladding, and shoehorned into the Airport slot of a G4 AGP.
Surprisingly, it works fine. I wonder if this will work with broadcom chipsets for Airport Extreme (802.11g) speeds? I suspect yes.