The cablecard Tivo boxes like the S3 and Tivo HD can decode digital channels and output them to a non-HD TV.
On the other hand you can buy an S2 box pretty cheap right now, and then you don't have to pay your cable company an extra fee for the cable card or outlet (assuming they charge you those).
On the other hand you can buy an S2 box pretty cheap right now, and then you don't have to pay your cable company an extra fee for the cable card or outlet (assuming they charge you those).