It depends what game and resolution you play at. That said, with the prices DDR3 are at right now, it would be silly to buy less than 8gb. Are you looking to upgrade or buy new?
To answer your question, 4gb will most likely handle games fine, but 8gb would be better. I don't see more ram adding to FPS unless your becoming close to maxing out your resources. If you're running BF3 and you have 1.5gb free ram, adding 4gb will do nothing for performance. If you're running BF3 and you have 200mb system ram left, you will probably lag quite a bit as dips and spikes happen with system resources while running games and other apps.