Real-time traffic monitoring router / firewall device

Solution 1:

I'm going to advocate Pfsense again. PFsense can by default graph outgoing / ingoing bandwidth. If you add the Bandwithd plugin, you can graph the traffic coming out of each node of your network on a per-IP basis. It even displays the type of traffic.

It's really simple to setup and works very well. You can put in transparently on your network by putting it in network bridge.

Solution 2:

if you have netflow capable devices use it. although it originated at cisco many hardware vendors support it. if it´s too taxing on your hardware use sampled netflow.

but most of all get a netflow agregator like http://nfsen.sourceforge.net/ for accumulating and visualizing the data.

Solution 3:

I believe smoothwall offers built-in graphing via MRTG. Not sure if it will do reporting via internal IP's out of the box. I think there are some other forks of this project that do similar things, but i can't remember the names.

ntop is probably more flexible for your needs. You should be able to run this on any linux distro that you can purpose as a firewall with iptables.