Note, however, that only the FW800 MDD machines take Airport Extreme cards. The other AGP G4 towers (every other G4 tower save the original 350/400MHz "Yikes" machines) only have a slot for the original Airport card. However, with OSX 10.3 or newer, the original Airport cards can handle WPA, so they're still useful. (no WPA2, though).
To be clear, Airport Express is the $99 WAP that you can hook up to a stereo/printer; the wireless cards are the Airport Card and the Airport Extreme card.