More often than not RAID is used for fault tolerance on servers and stuff. Gamers alot of the times have RAID 0 set up, as it does increase performace.
There is a way to set up RAID on OS's already installed, whichever raid you want. On raid zero, with the right program, it would simply copy yhe old drive to the new one so you can have the mirror. If you setup a spanned set, you can get programs to span it across the drives. It is possible, but not terribly easy. I would do it from a clean install. Format the drives, setup the raid, and then install Windows. Probably would run a lot more stable.