Your display adapter uses "shared memory" - which it takes from the
installed memory in the computer and re-dedicates it to use for the display
adapter.  There is no way to add more memory to one of these cards, though
you can adjust the amount of memory it uses on some computers in the BIOS
(hardware configuration) settings.

Most likely the only way you'll get newer games to play on your system is to
add an AGP display adapter and disable the onboard display adapter your
system came with.