Video ram is Video ram, you cant expand Video mem into RAM as far as I know. OpenGL and DirectX (The most common card techs) 'almost' do this by using system ram if the vid mem runs out, the two areas are NEVER spanned into a single unit though. You could program this effect very easily, just detect if you run out of vid memory and then create whatever you wanted to create normally.

Vid mem is more desirable as the access is faster and it is on the same chip as the GPU so it makes more sense to use vid mem whenever possable. System RAM however i comparitivly slower for use in graphics operations as the data has to travel around the MB and back instead of 'next-door' to the vid mem zone. If you need more vid mem then you would require a GCard with a higher capacity

Some integrated graphic cards do share ram with the system. Though I wouldn't know if its possible to do it with an agp card.

Bit hard to replace the graphics card in a laptop though.

I'm not sure how the Nvidia mobile chipsets work, but I imagine they have their memory to play with, which means you can't exactly allocate more system RAM for them to use (unless it already does draw from system memory, in which case the option for it is located in the laptop's BIOS).

There is the AGP Aperture size though - this is the amount of system memory available to the AGP bus should the graphics card run out of onboard memory. It should only be set to around 64-128Mb though - increasing it doesn't help very much since it is much slower to access as explained by 1o0oBhP.