The router should make no difference between your computer and the Tivo - it should route traffic from either out to the Internet, but the Tivo should then be plugged into the router, not the 2nd ethernet card on the computer. In this case it is your router that holds the IP-number that your ISP assigns to you and the IP-number of your computers network card is an internal one (static or given you by the router through DHCP) and the router NATs it for you. It will do just the same for the Tivo.

If you had, for instance, a simple (non-router) ADSL modem (or POTS modem) attached to the computer (to the 1st network card) and the Tivo attached to the 2nd card, then you'd need ICS. In this case the first network card holds the IP from your ISP and the second card an internal IP-number. You then need ICS to do the NATing of any internal adresses.

/Michael