If you want a linux "router", take a look at IPcop. www.ipcop.org.
This is a linux distro which comes with the very basic kernel and then uses iptables for the actual router. The cool thing is that IPcop comes as turnkey solution firwall router. You grab that 150mhz pc that you have. Pop in either 2 or 3 network cards. You know 1 card for Internet interface, 2nd for DMZ, 3rd for internal network. You could also just do 1 and 3 if you dont want a DMZ. But anyways if you are looking for somthing right out of the box but that is also ver customizable. Take a look at IPcop. I use it at my office and I have about 60 users and I run 11 servers. I have been running it for about 1 year now and it runs great. Like i said before, if you are just starting this software will work right out of the box and as you learn you can customize as you please. The website will have the iso image and all the documentation that you need, and best of all its free!