Please HELP! I don't want to have a chance of loosing any files!
First, you should do just a little bit of research to get a proper grasp on what your looking at.
Wikipedia gives a good, brief run down of what raids are:
http://en.wikipedia.org/wiki/RAID
Check out raid levels. For non-technically minded, a "nut shell" description of the RAID types most commonly used in home/office type setups:
Raid 0: best performance when dealing with large files. Zero protection. One drive fails they all fail
Raid 1: Mirroring. This basically makes a copy of every file on one disk on another. It protects you from total data failure if a single drive fails.
Raid 5: Provides protection against single drive failure.
The advantage of RAID 5 over RAID 1 is the in-to-out disk space ratio. For example, in RAID 1 you can only use 50% of the disk space you put into the array. For RAID 5, you get approximately 2/3's or more drive space back. (e.g. in a 5 disk raid 5 you'd get 4/5's of the available space back)
Of note, RAID 5 is *not* RAID 0+1.
Finally, as mentioned before RAID arrays protect you against catastrophic disk failure or losing all your data because a disk dies. It does not protect you against "oh ****" situations, such as when you accidentally delete your project directory a special kind of backup called an 'incremental backup' protects against this. Also, neither a backup nor RAID provide a solution to the "hmm, I need the project files from 5 years ago". This problem is solved by a particular kind of backup called an archive.
The ideal solution protects against both these situations and may involve, for example, using backup software such as Retrospect or free solutions like BackupPC to back up to a RAID 5 array of disks.
All of this may seem overwhelming, but there are a few VERY simple and NECESSARY questions you must answer *before* you even think about the solution you need:
1. How much data do you have to back up?
2. By how much does this data grow at what pace? (increases by x gigabytes every y months)
3. How long do you want incrmental backups for? (Hint: Forever is definitely the wrong answer. )
Answering these questions provide the starting point for knowing what you really need for your backup system. For example, if it's taken you 10 years to build up 500gb and you only want immediate access to the last 6mths of work which amounts to 100gb of data (in case of accidental deletion). It may be gross overkill to build a 1.5 terabyte (1500 gigabyte) RAID 5 array. A more appropriate solution may be an off the shelf 1TB mirrored array (with 500gb usable) AND a robust archiving policy for the 400gb of 'old data' that you save just in case or for records purposes.
Just some food for thought. Start with the 3 questions, backups aren't a penis measuring contest so don't let folks convince you you need a Mercedes when a Corolla would do just fine. The best way to avoid that is spending an afternoon on wiki or some such reading about raids, backups, etc.
Oh, I've always thought the backuppc docs provide some good plain talk backup basics in their docs. You may want to peruse those sections:
http://backuppc.sourceforge.net/faq/BackupPC.html#backup_basics