Welcome to MR
You'll find answers to this and many other commonly asked questions under the Guides tab at the top
Guides : Hardware: MR Guide to Buying RAM
Guides : Hardware: MR Guide to Understanding Intel Mac RAM
In general, it's recommended to have a Search in the forums and.or a look at the Guides before posting a new question.
Plus, do you need more RAM? For average usage, 2GB is more than sufficient. Are you planning to use memory intensive programs?