At 8TB of anticipated storage, you should be looking at a low end SMB storage product, not consumer grade stuff.
One of these would be a good place to start:
http://www.promise.com/storage/raid_...rsn1=3&rsn3=43
You want at least one more drive bay than you need (eg. 4 x 2TB is 8 TB on 4 drives, but you want a 5th drive as a hot spare - rebuilding a 2TB drive takes a long time, and you are vulnerable to failures in the interim)
You then want to stock it with enterprise class drives. You will thank me later for the $30 extra you spent per drive (and I'll tell you why you want these if you care, but the reasons are sound):
http://www.memoryexpress.com/Products/MX43655
That model drive there used to be called the Constellation series, and they are extremely solid.