***edit***
my bad, the solution below is not OS X, runs in classic... sorry
Sounds like you want to use one Mac as a proxy server. SurfDoubler from <vicomsoft.com> does the trick for 50 clams. It will share a dialup connection with up to two other computers, Mac or PC. You will need to have the computers connected in an ethernet LAN, either directly or through a hub.
Another solution would be a LinkSys router and an external modem, but the first way is cheaper.
Vicomsoft used to have a freeware proxy server, but I only see the pay4 version now. There may be freeware proxy server programs available, anyone know?