Comment Linux Router Project? (Score 1) 150
I'm surprised no one has mentioned David Cinege's Linux Router Project yet (LRP), as it would be ideal for this task.
It fits on a single floppy and will run like a champ on any Pentium class machine with 12-16MB of RAM. If your mobo has PCI slots, I would splurge on a couple of 3c905-TX (one of WAN, one for LAN, rock solid 2.0.36 module) and you're all set! I even run the Junkbuster proxy on mine.