Router Solution
 

what is the best way to set up a router box

I could use a full linux distro, BSD, or a custom linux like smoothwall or freesco

what I wan't

easy configuration and update
os that runs on old pc
long lasting solution

there are lot of router projects for linux. they run on all sorts of hardware even from a floppy. do a google search

zebra
 

Right now I've got a linux network using zebra. My pcs are running on Red Hat 7.3.

Basically just look at the requirements for zebra, gated or some other linux routing project and pick the least demanding OS for them. If you plan to do your own developement though, then you'll also need gcc, etc.

Zebra supports RIP, OSPF and BGP.

I suggest OpenBSD. It's not that easy to configure but it's as secure as nothing else. A router NEEDS to be secure ...