The hard drive trick can help, if you have them on separate controllers. This occurs naturally with SATA (unless you try really, really hard to make it not...) since it's a serial point-to-point interconnect, as opposed to IDE which can have 2+ devices attached to a shared cable/controller.

Or if you're blowing cash for Raptors anyway, just pick up one of those $100 SSD drives from OCZ or whoever. The SATAII ones should have more than enough bandwidth not to care if 5 clients are all pestering it simultaneously, and it will hose anything short of possibly SCSI as far as the many-small-reads that tends to be an issue for MBing WoW.