I think that a lot of people are confused about RAID. It is a redundancy system, not a backup solution. Think of it in terms of corporation. They use RAID and have backups. The purpose of RAID is so that if a drive goes bad, their server can continue to operate until a new drive can be loaded (and the array rebuilt). They also use off-site backups, so that a catasophe at the office will not take down all their data. Unless you need to have access to that data at all times, and cannot handle any interruption, I wouldn't use RAID. It adds a lot of complexity, especially if you don't know what you're doing. Hardware RAID is easier, but then you're looking about US$130 for a card.
Here's what I do, and I like to think that I'm very serious about backups. I have a 1TB drive in my HTPC (encrypted). I have 2 1TB drives (also encrypted) that I use for backup. They are kept in a safety deposit box. On Thursday, I retrieve one of them, and use rsync to make a backup. On Friday, I return the drive. The next week I use the other drive. This way a drive is always kept safe at the bank, and that data is only a week old. The failure scenarios boil down to two situations: drive failure and catasrophe. A drive failure means that there is a problem that only affects a single drive. However, they could all have drive failures at the same time. Really bad luck but it can happen, and there's nothing you can do (the more backups/redundant disks the less likely). A catasrophe is where something happens to the environment that the drives are in (i.e. fire, flood, stolen, etc.). This is where my system shines, and why corporations use off-site backup. A catasrophe doesn't matter unless it can take out a bank and my house.
RAID will protect against drive failures, but it also introduces another failure point: the RAID card. I've never used one, but I have heard that if the card goes, the array might not be salvageable.
Here are my reasons to go for backup instead of RAID: 1) easy. no configuration. You can make it as complicated as you want (cron jobs that use rsync) or just drag and drop folders in the file browser. 2) Cheaper. No RAID card. 3) Safer. How important is your data. Mine is probably worth more than anything else I own. There's nothing like off-site backup, or at least disconnected from the computer (small fireproof safes are only about US$25).
If you do decide to go with RAID, the more drives the better. RAID 5 is the way to go. I would look at using all 4 drives in RAID 5.