leandrodafontoura has an excellent point that backups should be done. It has become a catchphrase here - "RAID is not a substitute for backups."
But any RAID with redundancy (above zero, which is not really RAID, since the R stands for "Redundant") will improve the reliability of your system. A single failed hard drive will not affect the system's usefulness.
My _personal_ choice would be RAID 0+1, using four drives, plus buying at least two more drives for backups. If you only have one backup drive, and you are doing a full backup, and your system crashes, you now have zero backups. I have about eight drives that are used for backups.