Ideally,
Primary Master: System HDD.
Primary Slave: None.
Secondary Master: Second HDD.
Secondary Slave: DVD-ROM.
Most used drive (system drive) should always be Master on Primary channel. Then leave the Primary Slave open if you have only two more drives. Then install the other two drives on Secondary channel starting with the most used drive as Master (usually the second HDD). If possible, always try to use one HDD per channel.
If you add a fourth drive, install the less used drive on Primary Slave.
<font color=red>If you feel like going to work today, just lie down and it'll go away.</font color=red>