If I want to add traffic monitoring (e.g. bandwidth by IP over time) what
tools do folks suggest?

Maybe net-snmp together with mrtg (perhaps with some of it running on
another server)?

Any ideas / suggestions / paths NOT to take?   All with a view to
incorporating into m0n0wall of course :-).  [Hmm, we're going to need
optional installable packages real-soon-now!]

Ideally I'd like to keep it all self-contained in the box, and provide, say,
publically viewable web pages with simple graphs of the last 1-2 hours of
usage (thereby letting users answer their own question - who is hogging the
link?), but for now I'd settle for a text file of bandwidth usage that I can
munge on another machine!

Sorry! I realize this is a bit like asking 'how long is a piece of string?',
but throw me some leads and I'll track down the details :-).

thx, michael