[vox-tech] Measure network usage?

[Shawn P. Neugebauer]

I have a few Linux boxes that have uptimes of days to months.  I need
to try to estimate bandwidth usage for a long-ish period of time (e.g.,
days or weeks) in order to characterize how much bandwidth I use
(to decide on level-of-service issues for a new ISP---I have to move :( ) .  
Is there a way to tell the amount (in bytes) of traffic sent and
received by a running box?  Is there a simple *non-intrusive* tool that might
"add a little value" to whatever is built-in?  I'm aware of MRTG, and Orca,
but these are overkill for this type of problem.

shawn.