Linux router software recommendation?

[Robert La Ferla]

I need to set up a (free/open source) NAT firewall and am looking for 
recommendations.  I am quite familiar with iptables, etc... but it looks 
like there are complete packages available like FreeSCO, Smoothwall, and 
LRP (no longer being developed), etc..  I think it would be better to 
just install a dedicated Linux system for a router than a generic Linux 
distro w/iptables.   The hardware config is a Shuttle XPC (Intel 
Celeron) with one Intel Pro/1000 MT "dual port" server adapter (2 
ethernet ports) and a cable modem.  Looking to serve about 20-30 client 
systems including NAT, DNS, NTP firewall and DHCP.

Recommendations?