I know there are a lot of tutorials on the web about this, and there are even full distros (Clark connect, Smoothwall and others) dedicated to this, but Oreilly net have this tutorial and its part 2 on the subject. The first one is the main stuff like getting the server to be an actuall gateway, setting up squid, and a mail server too. the second one has things like Internal DNS servers, monitoring servers, and also monitoring traffic. this looks quite handy. im going to try this later on. check it out.