2GB is ususally plenty for XP Pro X32, 2GB kits seem to clock higher than 4GB ones too.
But if you feel you need more than 2GB then installing 4GB will give you access to 3.25ish depending on VGA
RAM, this will all be in dual channel which can make a difference to synthetic scores but not loads to real world results i think.
Correct me if im wrong fellas.