I do agree that your system could and can handle more RAM, the question is which RAM? Do you have DDR2 533 or 667? If you have 2 sticks of 256MB each at 533MHz, then it strikes me as more effective to go to 2 1GB sticks of DDR2 667 than to retain the older slower ones. If you dso luck out and have the faster RAM, then 1 or 2 sticks of 667 will work fine as this board doesn't seem to be dual channel anyway.
BTW slamming in extra RAM of either speed will help just not as much as simply replacing older slower with faster RAM.