also, I cant confirm this, but the gains you get from 2 ram sticks instead of 1 big one (the dual channeled part) go away once you add another stick (no longer dual channeled or something)... I read that somewhere, and I've allways stuck to it, unless I really need the extra RAM (i.e. needed for using art proggys or something).
IMO a better upgrade would be an actual video card, that will help improve RAM response, as there wont be constant accessing from the on board card. not to mention, having a video card helps reduce strain from your CPU, giving you more performance in that way also.
you dont need to get anything expensive, post your specs, and what you do with the comp, i can pry find one for a good price, or just suggest a few.