How do I check what is eating away at my bandwidth.

[Syd Carter]

Hi All.  Recently my voip traffic has been very susceptible to being 
clobbered by p2p file sharing programs operating within my network.  It 
is almost like as soon as a p2p session is started, the voip traffic 
gets choppy.  I've tried setting upload and download limits in the p2p 
programs, and changing to G711a codec.  What would you recommend as an 
application to use to analyze what is eating away at my bandwidth..  
Another option I would like to explore is setting up QOS via installing 
a monowall proxy.  Maybe that is the best approach and would save me a 
lot of hassle.