Very important update for your Photoshop version :

Do you have a Macintosh computer and more than 4 GB of Ram ? ...

On Macintosh computers, Photoshop can directly access up to about 3.5GB.
When there is more than 3.5GB of document data, Photoshop writes data to its scratch files as necessary.
On a computer with 4GB or less of RAM, the data is transferred directly between the scratch files on disk and the Photoshop RAM.

On a computer with more than 4GB of RAM, Photoshop tells the operating system to use the extra RAM as a buffer for the Photoshop scratch file.
In this case, when document data no longer fits in the 3.5GB of Photoshop RAM and is written to the scratch file, the operating system stores it in the extra RAM and can retrieve it from there much faster than it could be read from disk.
This lets Photoshop take advantage of more than 4GB of RAM to significantly increase performance with very large documents.

When this scratch file buffering is turned on with Mac OS X v.10.3 or 10.4, the operating system periodically pauses execution of Photoshop for up to several seconds at a time. This can cause problems when painting. Installing the plug-in causes Photoshop to disable the buffering so the pauses do not occur.
It also means that Photoshop only takes advantage of the 3.5GB of RAM it can directly access, so performance with very large documents will decrease.

For this reason, we recommend that if you use very large documents and have no problems with pauses during painting, you should not install this plug-in.

Photoshop Virtual Memory Buffering (dmg file, 20K)