When you run Photoshop CS3 on a computer with a 64-bit processor (such as a, Intel Xeon processor with EM64T, AMD Athlon 64, or Opteron processor) running a 64-bit version of the operating system (Windows XP Professional x64 Edition or Windows Vista 64-bit) and with 4 GB or more of RAM, Photoshop will use 3 GB for it's image data. You can see the actual amount of RAM Photoshop can use in the Let Photoshop Use number when you set the Let Photoshop Use slider in the Performance preference to 100%. The RAM above the 100% used by Photoshop, which is from approximately 3 GB to 3.7 GB, can be used directly by Photoshop plug-ins (some plug-ins need large chunks of contiguous RAM), filters, or actions. If you have more than 4 GB (to 6 GB), then the RAM above 4 GB is used by the operating system as a cache for the Photoshop scratch disk data. Data that previously was written directly to the hard disk by Photoshop is now cached in this high RAM before being written to the hard disk by the operating system. If you are working with files large enough to take advantage of these extra 2 GB of RAM, the RAM cache can speed performance of Photoshop. Additionally, in Windows Vista 64-bit, processing very large images is much faster if your computer has large amounts of RAM (6-8 GB).