A Router (there are many) that will connect to your cable
err why wouldn't you just use the Linux box as the router?
Connect the cable modem to the new Linux box and then connect the old box to the new with a cross over network cable.
Set up iptables on the new box as a firewall and forward/masquerade for the other box depending on how/where the ip address for the cable modem is allocated.
P.
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