Is there a way (software) that will help me to keep track of how much bandwidth each customer on a linux dedicated server is utilizing? This way if a customer goes over I can easily let them know? Any comments/suggested is appreciated.

The easiest route is just to give all of your customers unlimited bandwidth ::ducking and hiding from jim:: :D.

If you plug your dedicated customers into a managed switch (and if you are not you really should), activate SNMP on the switch port, and set up MRTG (http://www.mrtg.org) on another server.

This will monitor the bandwidth in and out of the server.