Have you already gone to you preferences in Photoshop and set how much memory you want it to use?
Make sure and read this:
http://kb.adobe.com/selfservice/viewContent.do?externalId=kb401089&sliceId=1
The memory information you are looking for is near the bottom of the page.
Here is the info for everyone to enjoy:
Allocating Memory above 2 GB with 64-bit Processors
When you run Photoshop CS3 on a 64-bit operating system, such as Mac OS X v10.4 and later, Photoshop can access up to 8 GB of RAM. 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, and actions. If you have more than 4 GB (to 8 GB), 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 increase performance of Photoshop.
The default RAM allocation setting in Photoshop CS3 is 70%. These settings should be optimal for most users. To get the ideal RAM allocation setting for your system, change the RAM allocation in 5% increments and watch the performance of Photoshop in the Activity Monitor. You must quit and restart Photoshop after each change to see the change take effect.
The available RAM shown in the Performance preferences has already deducted an amount that is reserved for the operating system from the total RAM in your computer. You shouldn't set the percentage of RAM to be used by Photoshop to 100% (unless you are using more than 2G of RAM) because other applications which run at the same time as Photoshop (for example, Adobe Bridge CS3) need a share of the available RAM. Some applications use more RAM than you might expect. For example, web browsers can use 20-30 MB of RAM, and music players can use 20-50 MB of RAM. Watch the Activity Monitor to view the RAM allocations on your computer.
Watch your efficiency indicator while you work in Photoshop to determine the amount of RAM you'll need to keep your images in RAM. The efficiency indicator is available from the pop-up menu on the status bar of your image or from the Palette Options on the Info Palette pop-up menu. When the efficiency indicator goes below 95-100%, you are using the scratch disk. If the efficiency is around 60%, you'll see a large performance increase by changing your RAM allocation or adding RAM.
Note: If you don't see the pop-up menu on the status bar of the image, widen the image window.
