Hi Eleanor,
May I suggest a trick that you might find speeds up things and makes use of at least some of your extra RAM.
Create a RAM drive - a virtual disk image in the internal RAM - then assign it as the first scratch disk in Photoshop. Whilst not as good as directly addressed RAM, internal RAM drives are very fast, much faster than a hard drive.
You can create the RAM drive manually - this page shows how -
Creating a RAM disk or you can use a utility such as
Make RAM Disk.
Note:
There are some RAM drive size limitations depending on which version of OS X you're using (2GB in Tiger although I think this has been raised in Leopard).
You should be able to create more than one RAM drive, with different names, and assign them as additional PS scratch disks if you wish. I'd suggest always having the final scratch as a hard drive just in case PS fills all the RAM drives.
Don't use too much RAM for RAM drives since if the OS is forced to swap active apps, such as PS, then it defeats the object.
RAM drives do not survive Power Down, Restart or being unmounted - all data is lost. This isn't a problem for scratch data of course. This does mean that a new RAM drive has to be created after every power down etc. I think the utility can be added to Start Up Items.
Finally, I haven't actually tried the manual method or utility above, however I have done this my own way using Terminal on Panther and with PS7.
HTH
Baz