BT has done an in-depth article on hard drive RAID arrays, you can read it over here.
"In the last few years RAID has become really quite popular. Once purely in the domain of high-end enterprise servers, today, any self respecting enthusiast motherboard had better have onboard RAID if it wants to be taken seriously. The abundance of onboard RAID controllers mean that it’s not unusual to see small arrays in today’s home computers. The reasons for this can be for increased speed, increased reliability or simply for bragging rights. After all, two (or more) disks are better than one, right?"