It probably depends on how many applications you have running at once. Unless you have a ton of them running at the same time I don't think you'll notice a huge difference if you upgrade to 4 GB. Even if you play a good bit of games 2 GB of ram should be more than enough at this point (in the future, who knows).
If you tell us what you do on your PC (video editing, serious gaming, or just routine web surfing/word processing) we'd be able to help you a bit more.