I installed 2 GB of RAM in my notebook, and the performance gain when I am, as usual, doing too many things at once (editing videos, blogging, putting together PowerPoint slides for lectures, reading email), is definitely quite noticeable.
The absolute best upgrade one can make today is to replace the hard drive with a solid state drive as their boot disk.
I gave this advice to a fellow at work a few months ago. Recently, he upgraded to a new PC which, of course, has an SSD. His response upon using the new machine?
"It's a beast!"