It's "more complicated" than that according to Jeff Schewe's article on The PhotoshopNews Blog <
http://photoshopnews.com/?p=134>:
Photoshop CS2How much RAM? Fact
"Adobe® Photoshop® CS2 will be able to go beyond the previous 2gig/process barrier that pervious versions of Photoshop were limited to. The answer of How Much Beyond varies. . ."
"On OS X with the supported system versions, Photoshop CS2 can go to 3+ gigs. The actual amount could be 3.5 gigs or so, but due to certain reporting issues, its tough to say precisely. Tiger (10.4) will not expand beyond this limit since at the root, OS X is still really only a 32 bit OS with certain 64 bit optimizations and Photoshop CS2 is still a 32 bit application for both Mac & Win."
"For Windows, the answer is more complicated. If you are running Win 2K or XP Home/Pro with SP1, the limit is still 2 gigs. With XP Pro & SP2, the limit is 3 gigs -IF- youve enabled the 3 gig boot switch, which can be problematic."
"If you are running the released version of Windows XP 64 bit edition and are running either an AMD64 or Intel EM64T chipset, the limit will be 3+ gigs or so with the same issue of reporting issues."