Router and Firewall with Linux and FreeB

I have DSL with 5 ips addresses. I would like to ultilize these IP addresses and as the same time create a secure home network. I heard great deal about creating a router in Linux or FreeBSD. I would like to have a Linux/FreeBSD box run as Firewall, Router. Which I assume the computer will have 3 NIC. One is Internal, External, and DMZ. I woiuld use one ips for masquerading to internal computer, and the other for mail, web or other servers. Since I have 5 ips addresses, what is best to do thisand how does it work with Linux or FreeBSD. Any layout with equipment and network design to make this work.
My plan is to have a dedicate computer(pentium 133) just to do routing and firewall, and another servers with mail and webserver.