It's a useful feature to have, though you need at least 2 hard drives, the capacities of which can be effected depending on the type raid array you set up.
I'm rather lucky here, and have 3 seagate drives set up in a raid0 array, so the computer spreads the writing and reading of info across all 3 hard drives at the same time, and thereby greatly speeding up the read/write performance. The down side of raid0 being that if one drive dies, I loose all the info.
You can set up other types of raid array too, like raid5 - which is like raid0 (stripping) but with data parity, the idea being that if one drive dies then the info it contained can be rebuilt from the other 2 drives. Though I had this before (raid5), but found that over clocking and BSOD's caused the raid array to always want to check its data for errors, hence slowing down performance when it did this, annoying frankly. So I reinstalled windoze and went with the raid0.