Western Digital Pulling Out Of SCSI HD Business
Actually, current IDE drives are about the same speed as curent scsi drives. The SCSI drives have high rotational rates (measured in RPM) and latency, and the IDE drives have much higher. Areal Density (loosely measured in GB per platter).
From my experience the best scsi drives are significantly faster than the best ide drives. My Quantum Atlas 10k transfers 25.1 MB/s when tested with 'hdparm -t /dev/sda'. The best I have seen out of an ide was 17.0 MB/s from an IBM Deskstar 22GXP.