Macbook Memory Upgrade?

Hi,
I just purchased and installed two 1GB memory modules into my Macbook (13 inch, white, mid-2007 2ghz, Mac OS X 10.4.11).

I know I've installed them correctly because I checked in the "About this Mac" section.
And now I have what might be a pretty stupid question: what exactly is this memory upgrade supposed to DO? I was running low on space in my Macintosh HD, so I purchased these memory modules. I have a lot of iPhoto and iTunes related files and just wanted some more space for them. But I've only seen a small increase in the area where it says "xxx GB available" at the bottom when you open the Macintosh HD window.

This upgrade should have literally doubled my computer's memory capacity, so why doesn't this reflect that? Am I just a complete idiot and bought this for nothing? Because so far it hasn't seemed to make that much of a difference that I can see, but maybe I'm wrong. I just installed it about half an hour ago and I don't know that much about computers.

Thanks very much for any help you can offer!