Cap the upload speed on ANY p2p programs you may be running. I've never come across programs that saturated the upload bandwith so much it lagged the internet, except for p2p. Google dumeter. It'll tell you how much bandwith the comp u install it on is using. It shows you the dl/ul on ur nic. It helps figure out why the heck ur ul speed is skyrocketing running certain programs.
The same company that has dumeter has a netlimiter program as well. Its not what u were looking for though. Whatever computer u install it to, it will have a cap of the dl/ul speed on ALL traffic through the NIC. Definitely not what you want to use for a single computer (maybe for a small group of computers cap their ul speed so when they use p2p it doesn't kill the internet).