well you set up an iptables firewall and routing on the linux box.
the firewall is only for forwarding incoming and outgoing connections (but of course securing your sytem too
)) and routing is for the computers inside the network to be able to find where to send the pakets to get to the internet.
there are plenty of tutorials over the net, and if you understand german, you should try the one at
raven.eplay.ch
that tutorial is from me. it is really easy...
have fun with it.
btw: if you need more help, you can mail me:
raven@eplay.ch
bye