When I lived in student housing, I had a 486 as a firewall to my computers. The most stable operating system I found for it was OpenBSD. I actually had an uptime on one installation of it for something like 320 days before the power went out. As I played with versions from OpenBSD 2.9 to 3.4, I documented a lot of the stuff I learned and the configurations I made. When I worked at The University of Texas at Tyler, I was even able to setup an OpenBSD machine to firewall for one of the computer labs. The operating system was more dependable than the hardware it was installed on.