and also darty, from personal experience, windows XP doesnt use any ram over 2gb effectively. i made the same mistake as u (ordered 4gb ram) and when i put it in, and looked at system specs, it said i was only using 2.3gb of ram. now thats a lot of waste, when i can spend about $300 less and get the same performance.
unless youre running windows server or x64, youre not gonna get the performance out of that last gig, and only .3 out of the other one.
also, looking towards the future, from what ive read, Windows Vista is gonna require 4-6gb, so you could use it then.... but it also requires a 4-6ghz processor, and a video card the likes of which is not even on the market