Bandwidth monitoring tools
Here are some tools provided by the Ubuntu repos for network monitoring:
iftop - display bandwidth usage on an interface by host
bmon - Portable bandwidth monitor and rate estimator, shows multiple interfaces at once
slurm - yet another network load monitor (has nice colored graphs)
tcptrack - A favorite. Tells how much bandwidth is being used and also what protocol (service/port) and destination the transmission is taking place to. Very helpful when you want to know exactly what is using up your bandwidth
Depending on the situation you can use one or the other (specific monitoring vs just statistics). For more information consult their man page as there are quite detailed information and examples