There it is. Straight and simple. How much ram can Microsoft XP home and Pro edition actually utilize? I have heard the answer to be 4gb from people I work with.

I have a friend that has 4 gig installed on a very modern mobo but he says it can only see 2.5gb of the total 4gb he has installed. He can then take out that ram and stick it into an older mobo and it will see 3.5gb which is that type of mobo's max ram anyway. What could be his problem with the new mobo only seeing 2.5gb? Thanks.