Otto, not sure what memory modules you have in your computer, but yes most likely you have 2 500mB Ram to make up 1GB. To get it to 2GB you will have to replace both of your modules with a 1GB module each; buy two modules to replace your two. The modules itself depend on your processor:
Here are part numbers for the memory modules
For use only with computer models using Intel processors:
1024-MB 434742-001
512-MB 434741-001
256-MB 434740-001
For use only with computer models using AMD processors:
1024-MB 431403-001
512-MB 431402-001
256-MB 431401-001
Hope this helps, good luck.