ntop is a great solution for this. However, you will need to either mirror the switch port going to the router or install a network tap.
You can find some liveCD’s with ntop on it. Check out the Network Security Toolkit.
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Since you need to monitor the bandwidth the best tool for this which is easily available and easy to configure is MRTG. As you are using Router,swicthes and firewall you can enable SNMP and the configure MRTG to monitor the bandwidth. It provides you a graph of bandwidth usage for every 5 minutes which can be also scheduled for any time (for for every 2 minutes). Here is the link for MRTG and the tools required. If you want more details please let me know.
Also you try cacti which is more efficient than MRTG .You can download from here & now its easier to install.
Also check my blog from more details on cacti.