In Photoshop CS5 and CS6, the History & Cache section of the Performance preferences also has three cache presets: Tall And Thin, Default, and Big and Flat. These options set your cache level and tile size based on the kind of document you’re editing and the amount of RAM you have.
Tall and Thin Better for editing images with smaller pixel dimensions that have many layers (dozens to hundreds).
Big and Flat Better for editing images with larger pixel dimensions (many tens to hundreds of megabytes) and fewer layers.
Default For images in between.