Marsha,
Irrespective if you go RAID 0, 1, 5, or 10... you will need a backup drive. RAID (of any flavor) is not backup.
Your first priority needs to be backup. Luckily... it is quite inexpensive to accomplish. With an iMac... you can directly attach a cheap USB drive. Performance of your backup drive really doesn't matter very much.
Next you need to decide if you want to keep your data internal... or external. You can keep it internal for a while while you decide. Depending on which program you use... it should be pretty easy to migrate your data to an external array.
I had suggested a Pegasus R4 or R6. You will not likely get better performance than that... but it comes at a pretty high cost. Either will be much better than a lower end two drive external array. Once again... you need to weigh cost vs performance.
I am personally buying the 768GB SSD with the intention of keeping my applications, user account... and my Aperture 3 library all on the SSD. I'll then use my 8TB Pegasus R4 (configured as a 4TB RAID 10) for all the other media other than my photography. I'll keep my iTunes media library, all of my video files, etc on the Pegasus. My configuration will be quite expensive... but it is a great tool... and probably about the best that I can do right now.
My primary backup will remain two-fold. I use a 3TB Time Capsule for local backup... and I use Crashplan+ for cloud backup.
I have a 3rd, (manual) backup strategy for my pictures. I have a rotating set of 1.5 TB HDDs that contain Aperture vaults and other media. One of the two is always off site in the desk drawer of my corporate office. This is by far the least reliable of my backup options... because it relies on human activity to back up. My goal is to back it up after every major update to my photo library (about every 2-3 weeks). In practice... I probably backup up and rotate the drives only about 3-4 times annually. Such is the situation relying on human intervention. Virtually nobody does it as often as they know they should. Those who are diligent... probably lie about it.
Hope this helps.
/Jim