PC Magazine reports: "How much memory does Vista really need? What about Windows XP? And what the heck does DDR mean? How much money will a memory upgrade cost, and where do you buy it? We'll answer these questions and more, briefing you on the essentials of PC memory technology and saving you a great deal of money and hassle. Whether you're building a new system from scratch or upgrading, you'll find PC Magazine Labs–tested advice you won't get anywhere else."
If you're the type of person who likes to understand everything, and you don't feel that you understand the different types of RAM that are available. Then I would recommend reading this article.
If you just want to buy RAM and install it in your computer and you don't care to have a technical understanding of it. Then go to a site like Kingston they make it pretty easy to buy it.
To learn how to install the RAM in your computer, watch the following video.