As Evongugg said you'll need at least two drives and the total capacity would be the summation of the capacity of the drives involved. As Evongugg warned there is no redundancy when it comes to RAID 0 - if you lose a drive there is nothing you can do (short of a professional data recovery) to get (some) of the data back. If you take evongugg's suggestion you're in the same boat though if you lose the drive everything is lost.
I personally run RAID 0 on my gaming machine as I really don't care if I lose the information or not - I generally wipe it every 3 - 6 months. It's cheaper to get 2 inexpensive disks and raid 0 them than put the money down for a raptor. You'll get 2x-5x the capacity and the same if not better performance in most area's. Just make sure when you set it up you configure it for what you need.