Venture into Photoshop’s preferences (Command-K) and you’ll find the Performance section, where you can adjust how much RAM Photoshop uses to load your image into memory. No matter how much physical RAM you have installed on your Mac, you should use the slider to keep the setting hovering around the 60% to 65% mark at the very most. This will leave enough RAM to run Mac OS X and any other applications you may be running, yet still give the bulk of available RAM to Photoshop.