If you work a lot in Photoshop with large files, be sure to read this article on Adobe about key steps to optimize Photoshop CS5 performance:
Allocate your scratch disk correctly
Separate the Photoshop scratch disk from operating system virtual memory
Set the RAM percentage used by Photoshop
Defragment your drives regularly
Minimize History States
Avoid “Out of RAM” errors when running memory-intensive features
Reduce the number of open files
Clear all available memory: Purge Undo, Clipboard, or Histories
Run Photoshop by itself
Optimize Cache Level and Cache Tile settings
Reduce patterns and brush tips
Optimize GPU settings
Choose the appropriate Photoshop file type
Set the Maximize File Compatibility option
Turn off thumbnail display
Reduce color bit depth
Deselect Export Clipboard
Adjust or turn off font preview
Optimize image files
