Don't expect to be wow'd by boot times or app launch times.
RAM doesn't necessarily speed up your computer as allow more things to be running at once. If she browses the web while running any other app, she's going to appreciate the ability to have more things running with less beach balls and perhaps some noticeable speed.
I remember when my mom called to complain about not being able to browse so many sites smoothly while using iPhoto. I installed 2x2GB RAM modules and haven't heard from her since. I know I should call to check up on her but, eh, she's got four whole gigs!!!