Thats a BIOS limitation then(or mobo hardware), Not OS. XP can effectively handle 3GB. Even before SP1.
EDIT: I use the term "effectively" very loose as XP becomes very innefficient with memory after 1GB. It's like it says "hey there is plenty, take all you want".
Vista can handle a boatload and also deal with multi-threading a hell of a lot better than any other microshaft OS.
Actually by his vagueness and your assumptions it equaled nonsense. He first stated the OS and mobo can play a role in mem capability. Then he later states his can't support more than 1GB(not indicating the reasoning). You falsely ASSUMED he was referring to the OS as the problem in his case.