If you're fortunate enough to have more than 2GB of RAM installed in your computer, Photoshop works a bit differently than it does when you have less than 2GB of RAMespecially if you have a 64-bit CPU and operating system. The more RAM you have installed above 2GB, the closer you can set the percentage in the Memory Usage percentage to 100%on a 32-bit CPU and operating system, Photoshop can't access more than 2GB directly, so once Photoshop gets its 2GB, the rest of the RAM will be used for the operating system and other programs you run.
If you have a 64-bit system and more than 4GB of RAM, things get quite interesting. On a 64-bit system, Photoshop can directly use 3GB of RAM, and Photoshop plug-ins can use the RAM between 3GB and 4GB. Above 4GB, you can get an unexpected speed boost: When Photoshop would normally move scratch data to its scratch disk, Mac OS X and Windows XP allow that data to move to the RAM above 4GB instead. We all know that RAM is much faster than hard disk space, so this use of RAM can really enhance performance. (Mac users should note the potential issue in the next section, however.)
What's the lesson in all this? If you work with very large files and are thinking about installing 2GB of RAM, think about installing 4GB instead so that Photoshop can reach its maximum RAM potential and still have RAM left over for the system and other programs. And if you edit large files on a 64-bit CPU and operating system, consider going all the way to 6GB or more so that you can take advantage of the faster scratch data storage in RAM, if your files are so large that this features applies.
Using More Than 4GB of RAM in Mac OS X
There is a bug in Mac OS X 10.3 and later that may cause a long wait cursor (that spinning rainbow wheel) when more than 4GB of RAM is installed. If you experience this, you may want to download and install the Disable VM Buffering plug-in from Adobe. The plug-in turns off the ability to move Photoshop scratch data into RAM above 4GB. If you have more than 4GB of RAM and you don't see the long wait cursor, you may not want to install the plug-in, so that you can continue to benefit from storing scratch data in RAM.
You can download the plug-in here:
Using More Than 4GB of RAM in Windows XP
In Windows XP, a program can use up to 2GB of RAM. However, if you have more than 4GB of RAM installed in a computer running Windows XP Service Pack 2 or later, you can add a line to the boot.ini file that will allow Photoshop to use up to 3GB of RAM. This is called the /3GB Switch. It isn't guaranteed to work on all configurations, which is why it's turned off by default in Windows XP, but if it works on yours, any program you have that is written to use more than 2GB of RAM take advantage of it.
You should try this only if you're familiar with how to edit the boot.ini file, because it controls how your computer starts up. Careful!
For Microsoft's information about this switch, read this Microsoft tech note: