I've got a couple of oldish computers, a 256 MB iMac LCD and a Toshiba laptop with 768 MB RAM but no hard disc. Both have only one ethernet connector but several USB (and firewire) connectors. I use a cable modem with ethernet normally, but it also has a USB connection.
Is it possible to use either machine as the basis for a firewall by connecting one of them ot the cable modem via USB then outputting to the main computer via the ethernet?