Shopping for RAM is confusing even for those of us who know their way away around our computers about as well as our houses. The quickest and easiest way to find what RAM your computer takes is to go to Crucial and use their guide selecting first your computer manufacturer then model (If you have a custom built PC you obviously know what RAM you need and this guide is not for you).
After you complete the questionnaire Crucial tells you what kind of RAM your computer needs and provides hot links to buy it. Here is where the sneaky part comes in: write down the type of RAM you need and then go to Newegg and find it there. Newegg is much cheaper than Crucial and you can even get Crucial RAM on Newegg for cheaper than you can on Crucial's site.