network/Internet monitoring software

I’m looking for recomendations on a good network monitoring software package. Basically, I have a 3 machine network that share an cable Internet connection via a DLink wireless router.

Our Internet connection is supposed to be a guaranteed speed. However, we see a LOT of variation - it doesn’t always seem to be as fast as it should be.

I’d like the software to track the activity from all three computers, as well as stuff like what ports it’s using, the IP addresses it’s connecting to, etc. so we can see if certain usage (such as on line games) cause slowdowns.

Preferrably, this software would be cheap or free.