Hmm...
The easy answer: Buy an inexpensive (non-wireless) Cable/DSL router and
put that between the Cable modem and the base station. Then hook all of the other computers directly to the router. No more software routing, and you have a really good firewall between you and the world.
Harder answer: use something like geeRoute to have the G4 route IP traffic between the Airport and Ethernet interfaces (I've never done this so I can't tell you exactly how to set it up).
However, I strongly recommend the hardware router. You will just have a lot less problems in the end, and you'll have a problem free connection forever.