I think SCSI is a good investment because the drives are built to run 24/7 for five years, and therefore have high reliability. They also have high performance, and SCSI allows for very high expandability too (up to 14 drives per SCSI bus). Even with SCSI, it would be smart to use at least software mirroring, or else hardware RAID1 (Adaptec 29320-R or LSI Logic 22320-R can do that for a reasonable price).
You might want to make the partitions about 5% smaller than the maximum possible capacity, so that if you need to restore a broken mirror and the replacement drive is of slightly lower actual capacity (36.1GB versus 36.2GB, for example), you have a little wiggle room so you can reestablish the mirror onto the new drive before adding a second new one and pulling the surviving old one.
[ preach ] You should also ensure that there's a very good backup system in place, where a backup of the important data can be stored off-site (tape drive, usually) in case a pipe bursts and nukes the server, or the building burns down, or someone breaks in and :Q steals the server :Q. Seagate has some DAT tape drives in the ~$650 range with 20GB native / 40GB compressed capacity. If you need a recommendation on a UPS, the APC SmartUPS 750XL is sweet (we have a couple at work). [ / preach ]