Find the installation guide for your particular make and model # of your motherboard. If you're not sure, carefully open up your computer, and look for a model number printed on the motherboard. It's usually near the manufacturer's logo..near the center of the board. Then go to the manufacturer's website and look up that motherboard model #. You'll find what type of RAM cards you need. You should buy as much memory capacity as the motherboard can support. 4GB is a typical value for a moderately recent computer.