For the price, just get it over with and buy 4 gig. If you only buy 2 now, in a month you are going to be thinking,
"maybe I should have bought 4 gig, I wonder if my computer would be better with 4 gig, why didn't I just buy 4 gig when I had the chance, I want to buy 2 more gig now and cannot find the exact same memory I already have" and on and on.
Just do it.
Your OS won't make use of all of it, but you can rest assured that now you have all the memory your system could ever make use of.