You really like to tinker. If anywhere, I would look into the BIOS. As I understand it, the graphics memory settings are defined in some Northbridge register. Question is: which one?

Lighthouse, I have had a little experience with the GeForce Go chips and I have to say that I'm not fond of them. They are usually used without any dedicated video RAM, so if you were to disable the shared memory there would probably be no graphics memory at all! Another problem is that Nvidia provides little, if any, support for these chips to end users. Usually, you get the drivers from the system integrator and you can't find new drivers from Nvidia. Imagine that -- a video driver that never gets updated. Criminal.