You can’t use four gigs of DDR RAM on 32 bit installations of Microsoft Vista. I recently got another two gigs (it’s very cheap right now) and was disappointed to see that Windows was only registering 3,326 megs. I do some research and find that 32 bit OS have a max of 4 gigs total memory it can use, and that includes sound cards, video cards, (everything) as well as sticks of DDR.

More details at CodingHorror.