Regarding how much memory is best for Photoshop, I found in my own investigation that it depends on the size of the image that you are handling, and how many different image processing functions that you perform on that image.
Photoshop keeps a history of each step that you perform on the image, and to do that it makes a new copy of the image in memory each time you do something. For ex&le, if you start with a 57MB 16bit file from a 2700 dpi scanner, then as soon as you do anything like rotate, levels, or curves, You'll see the "memory usage" jump by the size of the file each time. Adjust levels, and the memory usage jumps by 57 MB. Adjust curves, and the memory usage jumps by another 57MB. Eventually, when enough actions have been performed, the RAM is no longer enough to hold the next copy of the image and then Photoshop starts using the scratch disk for page swapping, which is when you'll suddenly notice that each new action now takes much longer.
When I scan a medium format frame I typically get a 200MB 16bit file. With 1GB of memory I can usually only perform levels and curves and image size before I run out of memory. Then the subsequent steps like unsharp mask and print preview become painfully slow because of page swapping to and from disk. I also tried 2GB of RAM, but it only delayed the inevitable by allowing me to perform a few more actions on the file before running out of memory. I would still nearly always run out of memory, even with 2GB RAM when processing medium format files.
In conclusion, the most important thing to enhance the performance of Photoshop is a high speed swap disk. I'm now using a partition on a RAID ZERO pair of hard disks on a PCI RAID card for my swap disk. That has made a huge difference to my Photoshop efficiency because now when I run out of RAM, which I almost always do, the next actions are still quite fast -- MUCH faster that just using the default system disk for swap space.
One more thing: Photoshop doesn't release it's memory when you close a file. You have to exit Photoshop completely before it releases the memory used by the last image.
Kind Regards,
Craig