Linux Tools for Internet Monitoring?

[Kirk]

I have recently put together an Ubuntu (full installation as available
online) machine that I have tested and is working.  I have never gotten much
past the novice stage with Linux but am competent.  Having said that, I am
responsible for an Internet network in three buildings (approx 150 users)
and the gateway modem to a VSAT system (1MB uplink/downlink, not shared).  I
am not surprised as we have continued to increase bandwidth that it gets
sucked up.  I am of the opinion that the nature of Internet behavior and
design of websites which include lots of graphics is as much to blame for
slow Internet performance as weather conditions or other unknowns that can
affect VSAT systems.  My request for advice has to do with what tools are
available, if any, on Linux that will allow me to monitor network behavior.
Can I tell if one machine is dominating bandwidth utilization on YouTube,
operating Skype or downloading movies/music/porno/whatever?  Can I tell if
one of my machines has been co-opted by malicious code and sending out SPAM
or anything else?  The VSAT Internet provider will send me very
generic graphs on usage but nothing of value.  I would like to be able to
respond with more information at hand when I get constant complaints about
"the crappy Internet."  If I know that something else is going on, I can
pinpoint my efforts at improving how we utilize the bandwidth that is
available.

I'll take all suggestions and would be happy to contact anyone offline for a
more specific conversation.  Thanks.

Kirk
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