I assume that you should have at least 8 gig of RAM and devote it to PS. It is based on the official Adobe Photoshop recommendations:
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.
I have not gotten my MacPro yet but I intend to get one with 8 gig of RAM--at least. I will get 2 gig with the machine and then the rest from OWC or some place like that at a much lower price.
You cant beat RAM for scratch disk space--as far as speed is concerned.
My 1 cents...
Ed Hopkins
baltimore