Network Bandwidth Usage tool

["betty Snoop" <betty111111@gmail.com>]

I have Debian 4.0 installed running as a gateway.  Command line only.

How can I log the ethernet ports for bandwidth usage?  Logging virtual
ports would be nice too.

Here's the situation.  I set up OpenVPN to allow remote users to
connect but I think the internet connection is not sufficient for what
is needed.   I want to periodically sample the networks interfaces for
a total bandwidth usage.  For example every minute I want to sample
both eth0 and tun0.

What tools are out there that simply log the total bandwidth usage on a port?