The AirPort Extreme Card can be installed in the following computers:

Power Mac G5
Power Mac G4 (FW 800)
PowerBook G4 (12-inch)
PowerBook G4 (12-inch DVI)
PowerBook G4 (15-inch FW800)
iMac - see this article instead.
iBook G4
eMac (ATI Graphics) (See Note)
eMac (USB 2.0)