It is better to have your largest drive as the primary disk where it installs the OS partition. This is because it uses this disk when copying data to the remaining drives. So if you have a 160gb disk, the max you can copy to the WHS at a time will be ~140gb. But if you use a 1,000gb disk, the max you can copy is 980gb.
Actually my understanding is that they stopped using the Landing Zone idea with one of the power packs. I would use 160gb for the OS drive as well.