it's a "feature" of directX 10 called Virtualized Memory. Since DX10 is only in Vista, that would be why you don't see it on XP boxes.
You can feel free to read through the Microsoft
White Paper if you want to know more about it..
The key point is that it uses system ram for video needs on as *as needed* basis. It does not reserve the system RAM exclusively for the GPU, meaning the "shared memory" value can be used by other applications and does not take away from your system RAM unless the graphics card needs more memory to run a task and even then, if the "shared memory" is in use by an application prior to the GPU requesting the memory, Vista will not release the memory for the graphics card.
edit: think of the shared memory as "it's there if you need it unless something else on the computer is using it first"
as for the original question on if the amount of shared memory can be changed, I'm not sure. It may be a function of the driver .. which would mean driver hacking.. I'll have to dig a little deeper and get back to you