If you have a spare computer you can also check out smoothwall.
There are a lot of easy hacks to block websites, ports, and even set when pc's can access the net.

Right now I am using 2.0 and have port forwarding used for 2 pc's, I have iptables setup to block several ports (mostly viruses) and I have acl rules setup to block a few websites that seam to cause problems.

Best part smoothwall is free. And very reliable, My smoothwall only gets turned off when it storms other than that its on on average 2 months without shutting down.
Put a switch behind that and you have a decent secure (funny word) network.