If you have a board that uses DDR2, you can probably put up to 2x2GB for 4GBs total in it. Its possible you have an odd board that won't handle 2GB sticks so you should figure that out before upgrading. I personally would buy either 2x1GB or 2x2GB and not use the 512MB. Odds are also good that the 512MB stick is slow, and will slow down the other sticks. It will also bring you down to single channel.