Both standard are rapidly becoming obsolete, IDE has been replaced by SATA and SCSI by SAS (Serial Attached SCSI).
Seagate has aready said it will stop manufacturing IDE hard drives as of the end of this year, but they may still sell them under the Maxtor brand now that they own it.
All major server hardware brands have now switched to SAS over SCSI, with scsi options only retained to support TAPE drives etc.
SCSI has a better command structure than IDE, SATA borrows some of this in the form of NCQ, SCSI is good for when you have large numbers of drive, as one controller can handle 15 devices per channel, as where IDE is limited to 2 per channel.
The reason I was thinking about going that route was the price of SCSI drives are alot lower that IDE drives.
That the first time I've ever seen that said, but if your buying second hand drives I can beleave it, as loads of older serves running SCSI are being replaced with Core 2 based Xeon setups with SATA & SAS these days. So there are probably lots of older small SCSI drives kicking around.
The Control is often the most expensive part with SCSI and if you don't have one the just forget it.
SATA is the future for the desktop, IDE is dead or soon will be.
Low-end Servers or large storage server have switched to SATA as well, With high prefromance or servers with high I/O needs running SAS.
SATA is your safest bet these day, drive space is cheep there fast enough for most people, if you want more speed there are the 10,000rpm Raptors from Western Digital, if you want reliablity WD also makes their SATA RAID additions which are designed for 24/7 operation.
I'm personally looking at 15K SAS, RAID-0 setup in the new year, but I'm a speed freak.