You won't need an additional WIC. What you might need is an additional ethernet port depending on what type of Cisco 2600 you have. In other words if you have a Cisco2611, it comes with 2 ethernet 10BT ports. A Cisco2610 only comes with one ethernet 10BT port.
If you already have a Cisco2611, then you are all set. Either connect the cable modem's ethernet port into a hub/switch or directly into one of the Cisco2611 ethernet port and use the other to connect to your internal LAN.