If the ISDN router will respond to SNMP queries (some can and do by default, some don't), then you should be able to pull many different pieces of information from it including bandwidth usage in each direction, ports, config & usage stats, etc. However, this will require some software or scripts on the PCs for generating and receiving the SNMP strings. Unless you like to write software or already know of a generic SNMP utility that can satisfactorily accomplish what you need, this might not be the best option...
However, if you want a plug-n-pray solution with, maybe a nice Web Interface, etc., there are network appliances available that will do it but they are generally fairly costly. Now, if you are already familliar with Linux (or are willing to learn a little), then you can build a box that will allow you to monitor every aspect of your network traffic, including content filtering and bandwidth control! This would be among the least expensive options. However, time = $$ so there is that too! Good luck!
Robert Pectol
http://rob.pectol.com/