SCSI controllers are also much more expensive than the free IDE controller already included on your motherboard. There is also a large premium in price for the SCSI hard drives themselves. Unless you have deep pockets, there isn't much of a point in going with a SCSI controller. SCSI cards also have the ability to have up 15 devices or more per card, while an IDE controller is limited to only 4 devices. SCSI cards allow these drives to be in a chain along the cable.
RAID arrays combine multiple hard drives to act as one.
You have to have at least two drives for a RAID array, and in some cases you need more. RAID is generally considered overkill for home users, although it is very important in the server market. For those really interested in high peformance, a RAID array may be worth setting up. The primary benefits of RAID include Redundancy and Performance, although not always both together. There are several different levels, or types, of RAID arrays.