Lloyd Chambers is one of my front-line go-to gurus when it comes to Mac performance tips. He has just posted an excellent article on Mac Pro performance over on his site (note also the special section on Photoshop optimization): http://macperformanceguide.com/
Be sure to navigate over to his blog and look around while you're there. Portions of Lloyd's site are subscription based and IMO worth the price of admission, but much of his info is free, such as this in-depth Mac Pro performance article. Thanks Lloyd!