As per your question elsewhere about my switching to an SSD based scratch disk, Photoshop can still make effective use of a scratch disk, and in fact, will create one regardless of the amount of RAM you have and whether you are running 32- or 64-bit. Photoshop needs something like 5-times the native file size for scratch disk, per image open. While it does much of it's performance in RAM, it's till saves your changes to a scratch disk until you save the final image. So, if you have even a few 50MB images open at one time, you can see how RAM/scratch disk get gobbled up quickly. Keep in mind that ever time you create a layer, use a layer adjustment, you're multiplying the size of the your image -- again, the reason Photoshop still makes use of a scratch disk.