Yes this a problem with Vista 32. It is not a problem with Vista 64 which the op has installed.
There is no setting that has to be changed. I run Vista Ultimate 64 with 4 gigs and it all shows fine.
It could be your motherboard doesn't support 4 gigs. However I doubt that is the case since it shows more than 2 gigs. I haven't seen a motherboard yet whith a 3gig limit. Usually it's in multiples of 2.