Macworld has a great FAQ about upgrading your RAM for a Mac.
The first step in upgrading your RAM is to decide whether you need to do so at all. Upgrading RAM doesn t necessarily help apps run faster (see Bang for your RAM buck below). But it can speed up the processes of opening or switching between apps something some of us do several times a minute. If those processes feel faster, so will your overall computing experience.
Upgrading RAM: FAQs [Macworld]