how to route proxied web traffic out alternate gateway

[gluker]

i have a squid box with one interface (eth0). i receive web requests from my network on this interface. i would like retrieve the web sites requested from a cable modem. i would like for the cable modem to be used for web traffic ONLY. All other traffic would be routed back thru the (eth0) interface. I have no problem adding another interface to this machine. I would like to now if anyone has done this before or has any brighter ideas for this type solution. if so, examples please? Any help would be appreciated.