Please remember a RAID is _NOT_ a backup. It is a way to protect against drive failure and the resulting data loss. Although your odds (of not loosing all your data) might improve using a Mirror RAID instead of having any backup scenario, over time you will become (IMO) sorry you didn't have a backup in addition. It's easy for things to spin out of control, and when they do the opportunity to make errors also improves.
I am using the MacPro with a RAID 5 Scheme: 4 disks with one that is left intentionally unused and ready to go to work in the event of a drive failure. Within that framework the remaining 3 drives have both stripped and mirrored partitions. All this facilitates the speed in which I can handle many different operations including system upgrades, software testing, backup and getting the best performance from my computer.
From experience I can tell you that unless you have a fairly sophisticated (such as, dedicated buses or controllers for separate external disks) you can run into problems. For example, if you used 2 - 1 TB external FW drives as a Mirrored RAID, and the Mirror RAID becomes "degraded" (let's say your cat unplugged one of the drives), your computer will take about 23 hours to recreate the Mirror by completely rewriting the data from the "non-degraded" disk to what was the degraded one after it has deteremined it's OK to do that (which in itself is nerve racking). If all your priceless photos are not backed up, you now have a single copy of them on one drive, and an operation of rebuilding that can go wrong. I had such a set up and over 3 days I was unable to rebuild, twice the rebuild process was interrupted by thunder storms (one night after another!) which took out the power. Unfortunately, there is no way to suspend the process, and my battery back up would not last anywhere near long enough! As you may know hard drives do not like to have their power go out in the middle of any process, so I consider myself fortunate that I didn't corrupt or loose and data.
I've probably gotten too technical. Just remember that TBs of data take a fairly long time to write and back up. And you have to get serious about protecting the setups and the data. If you, like many, upgrade using Apple's software update, you need to know that there have been upgrades to the System Software that have wreaked havoc with many 3RD party drives which stop functioning and usually need firmware updates. You probably know it, but the first thing they say to do before a firmware update is "back Up Your Data!" But at that point you are not backed up and your drive isn't functioning either!
I recommend going over to the Apple Forums and searching "RAID" as a topic.
Good luck, and so long for now, TOM
1. What advantages/disadvantages has a RAID 5 system from your experience
2. How easy is it to upgrade if I need 2 TB in the future lets say.