Photoshop performance expert Adam Jerugim has posted a detailed guide to optimizing Photoshop for peak performance. No single configuration is ideal for all types of work:
What type of Photoshop user are you?
[…] The photographer who regularly processes high-resolution images will greatly benefit from increasing the amount of system RAM available to Photoshop, whereas the designer who works with 3D models will obtain far better performance by installing a faster video card containing more video RAM. So, itemize the tasks that you regularly perform in Photoshop and then use the recommended setup details contained in this paper as the basis for optimizing your system.
Check out the guide for a discussion of how 64-bit, RAM, scratch disks, and more affect various operations. [Via]