Network Usage Monitoring and Blocking

We are wanting to find a program or a way to block certain web addresses from our users and monitor all their usage. We already have a Webramp for internet access so I think all we need is just ip forwarding on a linux box w/ two nic’s I can get that working ok. but I have no way to monitor their traffic and say they can’t goto certain sites. I’ve looked at some of the ipchains firewall stuff and I haven’t found any that do what I need.. I really don’t need a firewall just to be able to monitor / block the packets..

Thanks