I'll take a guess and say you have an Intel video chipset, or some kind of integrated video. If this is indeed the case, that's where the "missing" memory went. It's being allocated by the video chipset.
First, I do have 64-bit Vista installed. I have 4 GB of RAM installed and under Vista System Properties, it says there are 4 GB of RAM installed. However, when I look under Task Manager, it says 3.3 GB. Is this because of system overhead? Why is this happening?