Debian-Administration has made a small how-to on using Nagios to monitor your network traffic.
Read the usefull guide here!
Nagios is a powerful, modular network monitoring system that can be used to monitor many network services like smtp, http and dns on remote hosts. It also has support for snmp to allow you to check things like processor loads on routers and servers. I couldn't begin to cover all of the things that nagios can do in this article, so I'll just cover the basics to get you up and running.