I am ready to take the tivo plunge but would like to get some recommendations. I do not have directv and don't plan to get it for a while, so I will need a "stand alone" unit. What is the best unit to get? Is the price differentiation only due to length of recording availability or are there other feature upgrades between tivo models? If I purchase directv at some point down the road, will I be able to use this tivo unit with the directv or will I have to purchase a different model? Can I watch one program through cable while I record another? I have heard that you can't. Any other advice? Thanks.