You might want to do a little research regarding your G4 TiBook. I"m thinking the TiBooks came with a regular Airport card (11b). These cards cannot do WPA/WPA2 encryption. So you would have to either run WEP or unencrypted if you wanted the TiBook on the network. Alternatively, with the AExtreme, you could run a separate guest network that's unencrypted, or use ethernet.
Another option would be to use a USB-11g/n dongle for the TiBook, but something tells me the TiBooks had USB1.1.