sure - update the firmware on the airport card and you'll get 128 bit wep. if you are running mac os x 10.3 (panther) you will even get wpa too.
run software update and select all of the airport related items. you may want to update the other stuff too. note that you may need to run the updater a few times, as some updates require earlier updates.
it is also wise to run disk utility before updating to be sure there are no major disk problems which can potentially affect system software changes (and anything else for that matter).