The net meter in
MenuMeters gives an indication of one computer's usage for as long as the interface is active.
This wasn't enough for me, as we run three or so Macs through my router, and two of them are laptops that go between work and home networks. So I found
mrtg, which works with my Linksys router. It gives me pretty graphs of incoming traffic, and tells me that I'll be happy with the 8000 Active plan.

(however, as you'll soon learn, that requires you to be pretty happy with the Unix lurking in the bowels of Tiger in order to install it.)
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