Photoshop gets used in a huge variety of ways, from editing tiny icons laying out multi-hundred-layer Web designs* to wrangling gigapixel photos. The optimal settings depend on the work you do. Now the Photoshop performance team has posted a white paper on Photoshop CS5 performance, explaining various cache & GPU settings, discussing the impact of 64-bit and multicore, and more. Hopefully you’ll find it helpful.
* Web guys: Try the “Tall & Thin” option (yes, there is such a thing) under Preferences->Performance.