I think you're results are telling. Since the 18GB drive is so slow and hot I just wouldn't use it. Period.
Rather than go on SCSI, which is a bit expensive, I'd just pick up a board with Serial ATA (NForce2 anyone?) and use adapters on your drives to use them that way. SATA in combination with 7200/10K RPM drives is pretty damn fast. While it is arguably not as fast as SCSI, it should be adequate for your needs.
For cooling drives in a non-watercooled environment I would recommend rigging fans in the front of your case blowing in over the drives. I would also recommend trying to space them in your case (if possible) so that they are in every other slot rather than stacked immediately on each other. There are many cases by Antec, Chieftec, and others that are extremely tall (two removable drive cages, 6xOptical drive slots) that can accomodate you here, and they contain the mounting hardware to rig fans like I described.
If you are considering a custom build with that many drives, I would recommend a full tower case like the AX-01BLD that you can find here:
http://www.newegg.com/app/viewproduc...iption=&page=1
You will have plenty of room for all of your gear.
Using both IDE and SATA on some of the NForce2 boards you can have up to eight HDD/Optical drives (IIRC), so perhaps you may not want to go SCSI for that extra little performance boost ....