+1 to Patrick's advice.
Please consider what would happen in the event of accidental overwrite or deletion with a RAID mirror. Both copies would be deleted.
Also, your array is susceptible to natural disaster, theft, fire, et al. So it really is important to remember that RAID is not backup.
With that being said, my RAID solution is a 7x 3TB WD SATA3 in RAIDZ2 (similar to a growable RAID-6) NAS, connected via 10GbE for high throughput.
My backup solution is CrashPlan.
1. RAID is a drive just like any other drive with some special features. No drive by itself should be considered adequate backup. But any drive can be considered as a backup drive if so designated. The real problem with most RAID's is people buy inexpensive ones that take forever to restore, are highly unreliable, and more trouble than their worth. A quality RAID on the other hand, properly utilized, is a valuable tool.. much more valuable than a single disk solution.
2. Crashplan and other services like them, are just another drive. They are not backup solutions. They should still be considered as one copy of your data stored in one place and at risk from the same issues any other drive is. It's one basket to hold one set of eggs. Eggs are fragile.
3. To be considered "backed up" you need a minimum of 3 baskets to hold three sets of eggs. Remember, eggs are fragile.
4. How you set up each drive is just as important as the quality of your drives. For instance, a 8 bay RAID/NAS device might be capable of 5-6 different RAID schemes. Which one is right for you? Is your device protected by a quality UPS? If it's not, it's not part of your solution. Rule that device out, at best its' temporary. If you set up a mirror, does it mirror in real time or perhaps at the end of each work day? What is right for you? It probably depends on how you r other devices are set up. What quality of drives, the quality of the enclosure, the quality of the software that runs the enclosure, and the power protection for the enclosure. A lot of boxes to tick before it's called backup.
5. My personal system that works for me: High quality SSD drives for work drives. This is backed up at the end of each day to a RAID1. When the job of finished it's archived to the same RAID1 (2x3tb WD blacks) to my active archives which are roughly 2tb. An additional standalone 3tb WD Black is :"mirrored" at the end of each workday to the RAID1. So.. in my box there are 3 separate sets of my work, possibly 4 if I'm still working on it. This means my box needs to be able to hold 3 3.5 inch drives and 3 2.5 inch SSD's. If I had a new Mac Pro I'd need to change this. The box is protected by a quality server class UPS.
Then, at the end of each workday my RAID1 is backed up to my Synology 1813+ 8 bay NAS/RAID which I can transfer data to at about the same rate as my WD Blacks inside my box. It rebuilds one if it's 8 3tb drives in roughly 24 hours. It reads and writes both Mac and PC files. It's located in another room, on another AC circuit, and protected by a quality server class UPS. This is big enough to run a personal cloud, FTP, store my personal media, and it's separated from my active and inactive archives through designation.
Both my active and inactive archives (roughly 12tb) on my NAS/RAID are also backed up in real time via a 150mbps line to my Amazon S3 account. Amazon has one of the best server farms anywhere, and they own t heir own. It's not some startup company we just heard of yesterday who's renting servers hoping their business model pays off. Still, I'm thinking of buying a second Synology 1813+ and server class UPS to be located off site to take the places (and relieve the expense) of the S3. I'm currently working on getting permission to run a cable to my climate controlled storage facility where I'll then set up a second 1813+.. off site, the protections of RAID, all admin controls can be remotely accessed, and I'll put it in a dust proof safe secured t a shelf at about 10 feet just in case of that 100 year flood. All I need is the permission for the cable, but this is proving difficult as people sometimes try and live in these places so they're careful what they allow. But once it's done, I'll run it for a few months and then cut Amazon lose for backup purposes.
And finally: I watch for sales of 2.5 inch SATA HDD's. Any speed is fine, but they must be of a good quality. Each year they are labelled with that year and then that years data is copied to that 2.5 inch drive using a ESATA cable. This drive is then stored in a small fire/water proof safe rated for 60 minutes.
All that and I'm still not sure it's enough. The safe is setup as a faraday cage just in case.. Everything is automated so I only need to check if things are going as they should, except for the year end backup. My images are in other places such as on my website which is also backed up three times.. and on websites where I publish there are web size copies..