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off a switch port and "sniff" though to be honest I have always used it as an In/out single card proxy (not transparent) or as a transparent proxy. Not having tried what you are asking, I really can not say, but it will install on a P2, so any old box you have will work as a test box, and the iso is maybe 300Mbs to download... sounds like play time to me.

Some resources:
http://articles.techrepublic.com.com/5100-6350_11-5187742.html
http://ipcop.org/index.php?module=pnWikka&tag=IPCopAddons
But if you are fater detailed types of traffic you may have better luck with MRTG
http://oss.oetiker.ch/mrtg/3party.en.html

EDIT: Found this JD
http://demo.opennms.org/opennms/element/node.jsp?node=14
Had to dig a bit but it will monitor traffic TYPE by interface...
OPensource community model and paid subscription model.
http://www.opennms.org/index.php/Main_Page