You can find out how much bandwidth your linux machine is using with a simple tool called “bwm-ng”. In Debian, install it with

aptitude install bwm-ng

Then, just type ‘bwm-ng’ in the command line. It will give you something like this:

bwm-ng v0.6 (probing every 5.000s), press 'h' for help
  input: /proc/net/dev type: rate
  -         iface                   Rx                   Tx                Total
  ==============================================================================
               lo:           0.00 KB/s            0.00 KB/s            0.00 KB/s
             eth0:        2221.47 KB/s           48.13 KB/s         2269.60 KB/s
  ------------------------------------------------------------------------------
            total:        2221.47 KB/s           48.13 KB/s         2269.60 KB/s

Pressing the “h” key while it is running wil actually pull up a nice menu to change some of the options you are looking at.

bwm-ng is very basic, “iptraf” is another tool that provides some more functionality if you want to drill further into what is moving in and out of your box.