SCSI - faster than IDE. More devices allowed per channel. Better over all solution. More expensive though. But yeah changing from IDE to SCSI would change the performance... if your site deals with a lot of reads, like extensive database queries ect.
SATA still wouldn't come close to Scsi in performance. With SATA drive, they're still in the 8-10ms league, SCSI drive is in the 3-4ms seek time. If you're running any Database, SCSI had proven to us to be much faster
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I'll second Mfjp, SCSI is almost always faster than SATA although it depends what you are going to be doing with the server whether the extra cost is worth it.
I'm quite happy running Samsung Spinpoint SATA drives in Raid 1 for now, as they seem to cope fine with just about everything, but for contstant stressful tasks where seek time and transfer rate are highly important then you may be better paying the extra for a good scsi drive.