We just became Tivo addicts, and we have a DTV/Tivo unit. I can't remember how many hours but not more than 40, for sure!!
(I was thinking 25.) This has been plenty for us.
When you set your "Season Pass" for recording your favorite shows, you can tell it to keep only 1, 2, 3, 4 , or 5 shows. We set ours to only 1 for weekly shows like "Everybody Loves Raymond" and 2 for daily shows like "The Ellen Degeneres Show." This way, if you still have last week's episode, it just tapes over it. Ours has never had to ask us if this is what we want, as we have set it up to do just that.
We have never filled ours, and we have Tivo'd several movies, and have tons of Season Passes (the kids have all their favorites on as well!!)
You also get to specify the order of importance of your favorite shows - that way, it deletes your least favorite first, if deletion becomes necessary!!! And if you do allow Tivo to tape it's suggestions for you, as WDWguru has stated (BTW, love your Arrowhead!), those will be the first to be deleted, your preferences come first.
Good luck, and welcome to the Tivo cult!!!!!!!!!!
