the RAID controller sorts all that out...as far as the operating system sees there is only one drive there. Defragging will always help. RAID 0 means that the data is striped across X drives. So if you defrag it, then the data will be consecutive on disk. This lessens the need for head movements on both drives, since the data is side-by-side on disk. It doesn't matter RAID or not. It should always help.
