In Preferences, set the the Used by Photoshop to 80%. Also, make sure under Saving Files, that you do not have Image Previews: Always Save checked. And finally, under Plug-ins and Scratch Disk, make sure you have the first set to Startup, the Second set to C:, the third to D: or however your drives are set up. This will allocate more memory to your machine while you are in Photoshop.