2gb is still plenty sufficient for most if running XP....if you upgrade to 3gb you might notice a bit more snappiness, especially when alt-tabbing out of games and such. Or, if you use photo or video editing software you'll definitely see an improvement.
As low as prices are right now it really cant hurt to get another 2x512mb or 2x1gb (whatever you do make sure to buy another complete pair to keep it running in dual channel mode)