I have an ADSL four port Netcomm of which I am only using one port. I have the router/modem plugged into a Linux Smoothwall server set to DHCP. My Linux server has two NICs in it. One for the router connected via CAT5 100mbs card and a second card connects to my home LAN (3 computers) all of which can use the ADSL connection. The trick is that the Router is set to DHCP and so is Smoothwall. I need to boot the router first the server second then the LAN computers after that so as all receive their IPs dynamically.
Works well and is secure from probing eyes on the outside.
I think you should be able to use a single port modem in place of the 4 port modem if you tell smoothwall that it has a fixed IP. Everything else I guess would be the same.
PS. Speed of the LAN cards is not critical as even a 10mbs NIC is faster than any ADSL connection. Faster NICS just help in faster file transfer and movies across a LAN.