You can make an ad-hoc network between most computers with 802.11 (B or G) cards however you have to have them both on all the time and each time one of them loses a connection (like falling asleep in the PowerBook's case) then you have to reconfigure it.
Having said that, I don't quite follow what you're saying. Are you trying to share a connection in one machine with another machine that also has a WiFi card?
Maybe just buy a router, they're not very expensive if you keep away from AirPorts.