My point was that for his needs (single user playback of mp3 files and infrequent writes), maximum performance is hardly a requirement, whereas providing fault tolerance whilst maximising disk space probably is. Backing up/ restoring 300GB of media files is hardly trivial.

The hit on RAID 5 is parity calculation during writes..If course, the bigger hit is likely to be from using multiple drives on the same IDE channel - regardless of RAID mode.