RAID really has only three uses:
1. To enable faster access than a single drive is capable of, with not only zero redundancy, but an actual INCREASE in the odds of failure (RAID-0; although nowadays an SSD would be a better option if pure speed is all that matters.) Examples are scratch drives for high-speed-requirement processes like raw HD capture and database logging/index caching. Things where maximum speed is valued over all else, and downtime is acceptable in case of hardware failure.
2. To enable a single logical volume with higher storage capacity than is possible with one physical drive (RAID-0 or RAID-5; RAID-5 adds redundancy in the event of hardware failure.) But, ask yourself, do you REALLY need all of the capacity to show up as one single volume?
3. To provide minimum downtime in the case of hardware failure. Note, I did *NOT* say "to act as a backup". (RAID-1 or its variants, or RAID-5 or its variants.) This is the original purpose of RAID. To provide for redundancy in the event of hardware failure and still have the system stay up and running. For the vast majority of uses, this is overkill. You can live with your HTPC being down for the five hours it takes to replace a hard drive and restore a backup.
In all cases, RAID is *NOT*
*NOT* *NOT* a replacement for a true backup solution. If a virus hits you and screws up your data, RAID or not, it's gone. If you get a power spike that kills your computer, it will likely fry one or more of the hard drives in your array, and it's gone.
For an HTPC, the only reasonable RAID use is the "capacity expansion" one. For that, you can get one of the pre-made external RAID units like a
LaCie 2big, which will appear as one big drive.