Adobe has quietly increased the maximum RAM Photoshop can use in CS3! According to an Adobe technote I just found through Photoshop News, Photoshop can now take advantage of up to 8 GB of RAM on its own (over and above whatever the OS and other applications are using!). Photoshop's own limit for RAM dedicated to images is still 3 GB, but it knows two new tricks. First, filters and plugins can access approximately another 0.7 GB that the application can't get at (there's about 0.3 GB out of the first 4 available to Photoshop that contains Photoshop itself rather than an image). Between the 0.3 GB that contains Photoshop, the 3 GB it can use for images and the additional 0.7 that filters and plugins can use, it can use up the 4 GB that is the OS maximum for a single application.
Photoshop's SECOND new trick will let it use ANOTHER 4 GB of RAM if you've got it! It can cache the scratch disk in RAM between 4 and 8 GB. Since thrashing about with the scratch disk is one of the major ways Photoshop gets slowed down, this could be a huge benefit (forget about using Raptors for scratch, RAM is 1000 times faster). This suggests that the optimum RAM configuration for Photoshop (not necessary unless you are using huge files) is now somewhere between 10 and 16 gigabytes, depending on what else you're running alongside Photoshop (where it used to be no more than 6 to 8 in CS2). If CS2 wasn't RAM-limited on your present system, CS3 is apparently not much more of a memory hog. What CS3 brings to the table is the ability to throw more RAM at Photoshop if it was constantly accessing the scratch disk despite a lot of free RAM.
In order to get this to work, you need some fairly specific hardware (beyond "anything with a huge number of RAM slots"). It only works with a 64 bit processor (Xeon and Core 2 Duo, I don't know about G5 or AMD 64) and a 64 bit OS (Mac OS X 10.4 Tiger for sure, I don't know about 64 bit versions of XP or Vista).
-Dan
