Just a word of caution regarding OS software-based RAID (either RAID 1 or RAID 5):
As Satya notes, using OS-based mirroring or striping is a low cost solution. However, you get what you pay for.
If you really need to protect your data, you should consider hardware-based RAID solutions. Battery-backed, intelligent disk controllers may be more expensive up front, but that price will look small the first time your OS-based solution hiccups and corrupts your OS RAIDed mirrors/arrays.
I offer this advise based on painful, personal experience.