You can try, an easy to use solution as : ipcop, mandrake MNF, clarkconnect or whatever.
IPCOP:A secure Linux distribution managed through a web-interface. It turns an old PC into a firewall and VPN gateway. Features an Intrusion Detection System.
http://www.ipcop.org/Clarkconnect :ClarkConnect transforms standard PC hardware into a dedicated broadband gateway and easy-to-use server. The award-winning Linux-based solution includes firewall and security tools, along with file, print, web, e-mail, proxy, antivirus, antispam, content filtering and VPN servers.
http://www.clarkconnect.org/index.phpSmoothwall:SmoothWall Express is an open source firewall distribution based on the GNU/Linux operating system. Linux is the ideal choice for security systems; it is well proven, secure, highly configurable and freely††† available as open source code. SmoothWall includes a hardened subset of the GNU/Linux operating system, so there is no separate OS to install. Designed for ease of use, SmoothWall is configured via a web-based GUI, and requires absolutely no knowledge of Linux to install or use.
http://www.smoothwall.org/and many others solutions :
Mandrake MNF (Multi Network Firewall)
E-Smith SME Server
Sentinix
Freesco
Free-EOS
...
Generaly, this solutions are very simple to install and purpose a web interface to manage it. These software are especially creates to play the role of gateway, you can make a DMZ easily and activate many others services very useful. If you are a purist you can of course take a gentoo and compile yourself the only packages which you will need. The firewall rules are easy to edit and routing too...
If you laready using a Mandrake version, try to setup a specially firewall, gateway and routing Mandrake version...
Finally all depend of time that you can pass there and your level on Unix System
Hope this Help
dany