The graphics card requires the use of memory in a different manner to that used by an Operating System [Windows for example]
This is principally because of the function of the graphic controller chip and the 'instantaneous' requirement to present the contents of memory onto your display
In some PCs [notably Laptops and Celeron based desktops] an amount of System memory is 'shared' with the graphics controller to provide more memory
If you have such a machine then you may find in the user guide, how to alter the configuration to give more memory
Failing that, you will have to spend $ 100 or so for an average 64 Mb Video Card [make sure that it is compatible with your PC - some older units do not accomodate AGP and you'll need to buy a PCI card].