I've got an old 233 sitting around I'm going to use for that eventually - but for the moment, it's gotta be this. I'm out of power outlets in the computer room. :P
I'd get some powerstrips, but I'm unemployed, and pinching some SERIOUS pennies. :)
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche
Anyway, you've probably figured most of this out now.. but first use ifconfig to setup your WAN side nic with the info needed to connect to cable modem and then setup the DHCP server to your LAN side nic. That mini-HOWTO I pointed to covers the details and should get you going :)
Look for a Big Lots or Odd Lots near you, or maybe a Value City or some other discount store. They usually sell power strips for under $5 with a little surge protection. For $15 you can get that IBM model with four transformer adapters and four regular adapters. That is what my Windows 2000 Server uses and shares with my Linux server. I bought it before I become unemployed and was out on sick-leave. I got it from Big Lots. I think it retailed for $30 or $45 for that unit.
Linux is supposed to have some built in Firewall capabilities.
[link|http://www.linux-firewall-tools.com/linux/|Linux Firewall and security site]
[link|http://www.snort.org/|Snort]
[link|http://www.socks.nec.com/cgi-bin/download.pl|Socks5 Download] Socks is a favorite of mine and a few others. Just make sure that you have a Socks client on your host OS.