Monitoring wireless router

[quick-smart]

Hi, I'm new to the site and I wondered if anyone could help me with a question I have regarding wireless routers?

I have cable broadband which is used by 4 other PC's in the house via a wireless router. However, I connect via USB cable. I've been having problems with other users abusing the download limits and the service has been reduced several times during peak hours. At present my only option if this continues is to just block one or all of the users at peak times, however although I can see who is on the netowrk and who's downloading, I can't see how much traffic is being generated so I have no way of knowing who to block. I know there is software available to monitor this but so far the ones I've found have only been able to monitor my usage, no-one elses. Is there a way to monitor the traffic through the wireless router, and therefore to work out who is downloading what? The router assigns an IP address to each user, so I wasn't sure whether this could be used to monitor, or whether it would have to be done via the mac address.. although I could probably figure something out myself, it would save a lot of time if someone could point me in the right direction!

Cheers.