The Airport and Airport Extreme cards have entirely different connectors and shapes; you cannot simply replace an Airport card with an AE or vice versa.
If you want to upgrade the older machine to 802.11g, you need to by an external base station, either the Apple Airport Extreme Base, or one of a number of equally good but far cheaper alternatives.
These are not usually hooked up via USB, but via Ethernet.
For advice on which 802.11g hub to get, I suggest the Networking forum.
-s*