I just went with 5 hard disks (2 parity, 3 data), at a pitiful 750 GB per disk (..so a bit more than 2 TB). -this is just for data and game files, and I don't have much of either. The drives are 2.5" and after some discounts, cost about $33 each on Amazon (..at the time I bought them). I guess what's critical about my build in relation to the disks (other than not needing much space), is that my data drives are barely "spinning" and even the parity drives have it easy (with low use). (SnapRAID.)
It's still not a back-up solution.
..I should really do what BIF does with cheap 2 TB (3.5") drive and stick it in my (fire-proof) safe. (..critical data I do backup - to optical, and place it in my safe.)
-anyway, that level of redundancy on the ultra-cheap is one reason for purchasing smaller capacity drives (when you can get a great deal).
As for performance, it doesn't really matter to me - not in this context (use).
So I guess it's mostly a matter of *how* you are going to use your Hard Drive in relation to what you should be looking for.