[CentOS] Web Filter

I have a location using a CentOS 5 server that's multihomed running Asterisk and iptables for internal web access.

Recently some sales people got busted surfing some explicit content so the owner wants something in there to block this.

I had heard of Dans Guardian and am reading about what's involved here but just wanted an opinion on what's the best solution for this. NTLM silent auth would be an asset, but the lan is simple and the owner doesn't need granular control if it would be complicated.

What are you guys using with good results?/
Thanks?
jlc