You do not need a RAID 0 drive to work with Premiere Pro CS6. It will not hurt, but if you work on your internal hard drive, you should be able to get smooth playback with most files, as long as your system meets our system requirements.
A firewire external drive is not recommended because it cannot get the speed needed to really process these video files. Most video formats, if they are compressed, can be handled by an internal drive. It’s just when you get to the larger, less compressed files that you will run into problems.