I discovered how to watch the bandwidth of server today using a tool called bwm-ng. It’s in the Ubuntu Hardy repos, you can install with the following command:
sudo aptitude install bwm-ng
When you run it in a terminal without parameters, it displays a running update of how much bandwidth is being used. This makes it a handy tool to get an immediate idea of what your network traffic is like, and there are also options to examine disk IO. Besides the “live running commentary” mode, it can also output in other forms as well, for example CSV.