Just published is my revealing report on how a hard disk drive compares to a solid state drive (SSD) with large Photoshop files.
For anyone working with large Photoshop files, you need 8GB memory and a solid state drive like the OWC Mercury Extreme, and your best choice is the MPG Pro Laptop with dual internal SSDs as a RAID-0 stripe (up to 800GB capacity).
In the process of preparing this report, I discovered a bizarre performance issue: unless a Firewire 800 device is attached, internal SATA performance plummets. Seriously. Click the graph to read more.