Your computer will only support 4Gb of DDR2 667 also called PC 5300. Since you already have 3GB I really do not think it is worth it to upgrade. You would have to buy 2 x 1GB sticks and take out the 2 x 512MB sticks. It would be a cheap upgrade but you would probably see no real performance gain from it.
If you do upgrade then know that you can only use one 1GB stick of RAM in each of your 4 RAM slots. Buying a 2GB stick or any stick larger than 1GB will not work, so you can not just buy one 4GB stick for instance.http://support.gateway.com/s/PC/R/1015003R/1015003Rsp3.shtml
You can also use DDR2 800 also called PC 6400 as it is backwards compatible. It will however run at the slower speed of 667 due to your motherboard.