You do not need a 3rd drive to do RAID; just set up
RAID 1 (never RAID 0) so all data is mirrored between the drives. If one drive kicks the bucket, all data is still there on the other. You get a bit of a write penalty and a read gain, but the main benefit is the fault tolerance. Since the whole point of the second drive was backup anyways, losing access to half the total storage space shouldn't be an issue. RAID 1 will do a far better job of backing up data than you could ever do manually. Automation is the key to backups.
Yes, it is technically better to use RAID 3 or 5 in terms of lost percent of storage space, but its not strictly necessary for a desktop computer. A media center would be better off with more drives and a higher RAID level, due to all the massive movie files, but you aren't doing that I think.