Setting up a Simple Linux Firewall

I've been tinkering with setting up a Linux router/firewall and ran across this article explaining how to create a simple setup. I'm in the process of evaluating the prepackaged firewall/router distros right now.

Two-Faced: Setting up a Simple Linux Firewall: "First of all, ask yourself why you're doing this. You can buy a simple off-the-shelf firewall/router gadget pretty cheap these days, and I hear at least some of them have fairly easy-to-use administration features (e.g. via a web browser). Saving money by using that old PC you've got kicking around is unlikely. Acceptable answers include 'learning experience' and 'insanity'."