As it has already been said, the TIVO S3 will be your HD tuner. I have a 4 year old Sony HD TV and it does not have HDMI inputs, only DVI. If your TV does not have HDMI, you can buy a cable with an HDMI conector on one end and DVI on the other.
If you use component or HDMI to DVI cable, you will need to have the sound sent through the analog or optical output on the TIVO. I have my TIVO connected to a Denon 5800 via the optical connection, so the Denon can decode Dolby Digital soundtracks found in many programs.
My experience with CableCards installation was not smooth, though after several trips, several cards and lots of patience, it finally worked. After 2 years, having experienced the Comcast to TimeWarner switch, and not happy with TWC service, high prices, charge for everything and very few HD channels, I have decided to drop cable and just get my local over the air channels.
The moment I removed the Cable Cards the TIVO detected the change, asked to repeat the setup, and I was receiving OTA HD in less than 5 minutes! And do not forget, though there is only one antenna input, the TIVO still can record 2 channels simultaneously! Plus all my season passes moved to the new channels w/o any problems.
IMHO, OTA HD quality is better than cable.