This is mostly correct, but not entirely. You can do Raid 5 under OS X using the built-in softraid. This will not perform as well as a hardware assisted raid card but it doesn't require any reconfiguration of your MP. Just add a third drive and configure the array in Disk Utility.
That procedure will erase all the data on the drives in the array. So, you will need the use of a storage system with enough capacity to hold the data temporarily. Some users have been known to 'rent' the needed equipment from Best Buy for the duration of the project.
Finally, the risk of failure is actually a little worse than the number of drives doubled. You also have to consider that any software, controller, a\c power or emi problem that causes a mis-written bit to one of the drives will usually result in the complete loss of the array. If you've been running a raid 0 array for more than a couple of months and its still up you must be pretty lucky. I know I'm beating this point pretty hard but raid 0 is just such a bad idea in your case.
As was well stated in a previous post, raid is not a backup system. And raid 0 is never a good idea for a desktop user. The points Mainstay made about offsite backups are really valid and are worth some thinking. Unless the data you have is valueless, in which case the loss won't matter.
I know RAIDs aren't backups inherently. But I was backing up
to the RAID 0 because I needed the single large volume and it was all I had available at the time. I do know it's volatile, but as far as I can tell Apple's Disk Utility doesn't do RAID 5 -- only 0 or 1. I don't believe SoftRAID will do RAID 5 either.
But... an update for those interested.
Decided to bite the bullet and spend some $$ earlier than I planned. Bought from OWC a Mercury Qx2 RAID enclosure, four 2TB hard drives and a PCIe eSATA card. I configured the Qx2 as a RAID 5 across the four drives (6TB formatted capacity) and installed it on the new eSATA channel. I would have used one of the two additional SATA interface posts on the Mac Pro's logic board but each already has a drive).
I disconnected the file server from the network and use Disk Utility to clone all the data from the internal array to the external RAID 5 box and made a couple of configuration changes to OS X Server to point the shares to the new RAID.
I then installed SuperDuper! and set it to 3 times per week at 1am do a smart update from the RAID 5 box to the internal array (still presently a stripe). That process, tested over the last couple of days, takes only around 15 minutes each time.
So all the data is now primarily stored on the external RAID 5 box, and three times per week duplicated on to the Mac Pro's internal stripe for a redundant backup -- even though 98% of the data on the file server is backup data to begin with.
I sleep better at night knowing I have at least some kind of backup of the backup.
At some point, mostly depending on when my funds are replenished, I'll look at some way of doing weekly or every-other-weekly off site backups and rotating out the media somehow. Internet-based solutions like Carbonite aren't really feasible for huge ~3TB data sets. My iPhoto database alone is pushing 90GB and iTunes is upwards of 1.3-1.4TB. Another matching OWC Mercury Qx2 box w/ 4x2TB drives to do a Mirrored RAID 5 would be really nice, but at seven bills it'll have to wait!