It improves a lot, especially if you multitask. I just went from 128MB to 384MB and I can really tell the diff when I do web development (Flash, Dreamweaver, Explorer and Freehand at the same time). You know all that disk thrashing that goes on when you have a couple of apps open and you want to switch, that disappears. Also, if you work with an image editor, there will be a huge difference with big images, and even smaller ones.
One last suggestion, at the price of RAM these days, you should definetly pop for a 128 MB stick instead of 64.
-Ice