Nope, as other posters said, you definately don't need to raid if you have multiple drives, and I wouldn't recommend doing so either.
Hard drives are very cheap, you can pick up a 500GB for about $80usd, and on down the line. Many people that are into media stuff like to have a smaller drive for operating system(s) and software, etc. Then having a larger drive or multiple drives for media storage. Depends on what you already have I guess. Bottom line: I wouldn't use Raid, and do really whatever you'd like number and size wise.