One thing to take note of, I've never actually used WHS, only quickly tested it in a VM, but it's my understanding that you can't use RAID with it. Rather, it uses its own proprietary software RAID that mirrors data over a volume.
It is quite clever in how it works though (again, presuming I'm not getting mixed up). Consider the following:
You have 8 x 1Tb disks and WHS installed. You want 1Tb for downloads, but decide that downloads aren't important enough to warrant backing up, so don't turn on replication.
However, you do want your client PC backups replicated, and so you turn replication on for the 2Tb share in which you have them stored. This means that the 2Tb is mirrored onto another 2Tb dynamically. You then want another 1Tb for a general public share, with it not being replicated, and a 1Tb share for your movie collection that is replicated.
So in that scenario you would have used the full 8Tb:
4Tb for machine backups and replication.
2Tb for movie collection and replication.
1Tb general share
1Tb downloads share
Someone with more experience with WHS can probably advise you better
