i get the feeling your motherboard is NOT dual channel -- double check it actually is
xp wont benifit from well anything its a dead os you wont see much difference from 1.5gb - 3gb
windows 7 will love the higher ammount over a higher bandwidth, better still 2x2gb matched dual channel or not
rams golden rules (oldschool but still applies) -- least ammount of modules, most matched modules
try to stick with a matched pair
can you explain in simple and detailed please!!