I think I would go with this..
http://www.clarkconnect.org/
They make a free home version that acts as a firewall/gateway. Its has
a fairly automatic setup and will run on very minimal hardware. I use
their office version to host my websites and my mail server.
The setup for a cable modem is really very easy. You just select "cable
modem" from the set up menu and tell it to get an IP address from the
DHCP and your all set. It then runs its own DHCP for the computers
inside the firewall.
Its easy, its free. Its based on Red Hat 9.
Devin