The amount of actual RAM will likely limit how much RAM you can devote to the graphics core.
1st check if the BIOS will actually allow you to adjust the allocated RAM. If you can then, I would guess that you cannot go beyond 768MB unless you install more RAM because you only have 2GB of system RAM. Windows needs a certain minimum amount RAM to operate so in other words, you will likely have to install more RAM to be able to allocated more RAM to the graphics core.