i have the tivo 80hr dual tuner. i want to add a second drive.

i have available for the second drive- a seagate ultra ata/100 400gb drive 16mb cache, OR a WD 320gb 8mb cache. is one better than the other for adding to the tivo?

i already bought the twinbreeze kit for my unit and i'm ready to do the install. after looking over the included instructions, i'm a lil confused(read-noob'd).
what exactly do i need to do to the new drive to add it to the oem drive? is it not as simple as adding a second drive in my computer?

thanks for the help.