So what does this add up to? When you want to know the speed, you need to know what data transfer type you are using. Sequential ? or Random ? An average fileserver and printservers use Sequential transfer, which occurs most when you transfer big files in one go. Database servers, like Oracle, SQL or Exchange, use Random transfer. They transfer small chunks of data (a zip code being updated, a mail message of 2KB being transferred.
In a sequential environment, only the Latency of the harddisk really counts. So lets imagine, we are going to transfer 64KB Blocks to a harddrive, with a Latency of 3 Milliseconds. The harddrive can have a maximum of 1 I/O (input or output) every 3 Milliseconds, so in 1 second, that is : 1000/3 = 333 IO's/second. 333 * 64KB = 21312 KB/s, which is equal to 20MB /s. This harddrive has a peak transfer rate of 20MB/s. You will need to attach 16 of them to reach 320MB/s (Ultra 4 SCSI).