The way I see it.....See if you're satisfied with 2GB of RAM, first. If you find that it is sufficient, then theres no reason to go for more. If you're not satisfied, then go and buy another pair.
I have 2GB of RAM, and I'm completely satisfied. No lag. So I don't need to drop the money, for the 4GBs all at once. I can spend the extra elsewhere, or save it. Now if I see that I need more, with the use of memory intensive programs, then I'd go and buy another pair.