I'll echo what Darren has said about point 2.

Any 2 drive TiVo has twice the chance of loosing all your shows over a single TiVo. And marrying a new drive with an old one increases the chance of failure even more.

Unless you really need to store that many shows on your TiVo, you would be better off adding the 160gb to a PC and storing some shows there. At least if it fails you'll have better chance of recovering your data.

Peter