Not a dumb idea - actually a very good idea. I have done it on 5 or 6 Macbooks, the oldest a 2006. It is relatively easy to do.
Find your Macbook model (Apple Menu / About this Mac / More Info / Hardware .... The Model Identifier number (e.g. 2.1) is the easiest way to tell what RAM you need, and the maximum the MB can handle.
This is my RAM provider of choice, the site tells you what you can do:
This is another popular RAM supplier, the site has "how to videos":
Buy from a supplier who knows Macs and there will be no compatibility issues.