I'm thinking about three HD's. One for the OS, one to record onto and the third as a redundant back-up. I think Leopard already offers a feature that will back up everything automatically, correct? My only other consideration here is that I might want the backup HD to be external so I can remove it from the location when not in use.
As for the interface, I'm using the MOTU Traveler (can't remember the model), but it seems to be more than I need.