IDE and its Ultra friends are great for huge drives to dump crap onto, and even mirror. Keep the OS and the swap file on a SCSI drive, and you can use your CPU for something else. Trade CDs with everyone you know. Believe me, I've done it twice already, and even with backups it's still a huge pain in the ass to reinstall. ITs basically an extension of the ancient AT disk interface spec to encompass new features like DMA, auto-detection, higher raw data rates, removable media, etc.
Programmers are just machines for turning coffee into code.
I still tend to buy only SCSI disks. They cost more, but tend to have better specifications, in general. The manufacturers figure that people buying SCSI disks are probably building RAID arrays. There the high bus bandwidths actually help, unlike a typical desktop installation. So I could have 4 disks running flat out and the disks would still be the bottleneck.