You know I have experimented with FC3, but not with its firewall/NAT, but I know there's a special Linux distro that lets you convert you computer into a dedicated firewall. They have docs on well-known strategies, or network plans and diagrams that you can follow so you can secure your network. It is supposed to let you do all kinds of manipulations with ports and other network security things. This distro is called
Smoothwall. Check it out.