Is there a simple way to determine if adding RAM will make a material change
in performance?

I have 512Mb and do not do anything particularly unusual though I do edit
pictures (4Mb in JPEG format - obviously bigger when open in a picture
editor).

Task manager says 450Mb page file used at a typical usage of programs open.

Task manager also says under physical memory:
Total 523760
Available 146000 (but goes up/down)
Cache 248000 (but goes up/down)

Kernel memory says:
Total 56740
paged 44056
nonpaged 12684

By opening some more programs and 6 jpegs in paintshop pro I got the page
file used up to 580Mb.

Is it possible from this to say what optimum RAM would be?