Disks
While it's always possible to run into situations where the CPU in your database server is its bottleneck, it's downright likely that you'll run into a disk bottleneck—particularly if you only have a drive or two in the system. A few years ago, the basic two choices in hard drives were the inexpensive ATA (also known as IDE) drives used in desktops versus the more serious SCSI drives aimed at servers.
Both technologies have marched forward, and the current choice you're most likely to run into when configuring a database server is whether to use Serial ATA (SATA) or Serial Attached SCSI (SAS). It's possible to find nearly identical drives available in both interfaces, and there are even drive controllers that allow attaching either kind of drive. Combined with a narrowing performance difference between the two, picking between them is harder than ever.
The broad parameters of each technology are straightforward to compare. Here's the state...