You would have been better putting this in the Vista forum I think.
I know Vista is very memory hungry. Under XP you probably wouldn't notice much difference unless you used really memory hungry programs. Under Vista who knows? I think it would give a noticeable difference but I don't know as I only stuck with Vista for a fortnight and have no experience of upgrading the RAM under Vista and the boost in performance, if any.
Hopefully someone else will have done this and will be able to comment on his/her personal experience as to what it does.