virtual memory

By default, Vista allocates RAM dynamically. Like XP, if the page file is managed by the system, it starts at 1.5 X your physical RAM and the will expand dynamically up to 3 X physical RAM. It's not exactly 1.5 and 3 X though. I don't know the algorithm, but it's not precise. There shouldn't be any shared video memory as long as your graphics adapter meets the recommended specs, which can be seen at the web address below.

http://www.gartnerwebdev.com/2007/05/29/windows-vista-sure-looks-nice-but-wait/