Often the first thing someone will tell you when you mention that your computer is running slowly is that you need more RAM. While this may be a true statement, it is a good idea to verify for yourself before making a purchase.
Begin by actually using your computer. Open any programs you use frequently and might typically be running at the same time; maybe even open an Explorer window or two for a little extra. If you play a lot of games you might want to start a game and then Alt+Tab out.