just going to play devils advocate here but really, are your media files so important that you want to put up with the headaches of RAID?
If you're wanting to hold onto the data long term then you're not going to be stressing the disk in write cycles so the chance of failure is pretty low and conversely if you want to cycle the data frequently then it's not really that important so doesn't really matter if it does fail.
When you eventually fill your RAID and want to expand it you'll likely encounter issues. Instead of just adding a few more drives to your pool you'll have to set up a separate independent RAID and then copy all the data over. Which will mean you need another RAID card in order to have them both running concurrently to copy it over!
RAID tables which are vital to the array functioning are another vulnerability - I think they are duplicated on each drive but they can get out of sync and in the worst case can require a lot of effort and knowledge to repair them. If they become corrupted you can lose all the data on your array
So personally, I'd just get a single large drive. You can combine multiple folder locations to view as one in XBMC and you can then just add and remove drives as you need. After playing around with RAIDS, JBODs, LVMs (linux) this is the solution I've come to prefer.
If you're worried about data loss then go for an enterprise class drive. I think these have failure rates at least an order of magnitude better than desktop drives.