Not sure on the process of setting it up on an already installed Linux system, I guess it would mean setting up a DHCP server on there and using IP tables, but like I said, not sure really. If you have a spare PC lying around, which I know, not everyone does, but if you do you could install Smoothwall Express http://www.smoothwall.org/get/ , I run it on an old 266MHz PC with 64MB of RAM and it serves my 2Mb ADSL connection fine. It serves as a decent router with some decent security and has a lot of community plugins and updates too. Hope this helps out.
