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If you have a fedora system you can use the basic ethereal tools, this can be installed as an option with gnome.

Another option is as stated a tool like Ntop. Installed on teh gateway, it can tell you the usage of bandwidth by every interface and every client connecting to the internet. Ntop gives pretty detailed statistics, you can get it at their site www.ntop.org. You will however need to follow the links to get the rpm or source code.