You can, but I would suggest you to just buy 1 stick of RAM. It's cheaper then buying 2 512MB RAM's. What's the model of your RAM? To check you can either open up your computer, while it is turned off and take out the RAM. It should put the model number on the manufacturer's sticker. If you don't feel comfortable about opening up your case, you can download a program called CPU-Z, and check your RAM model through there.