when the graphics card hadn't that much memory, you could use a part of your normal memory for the graphics card.
It is advised to set it to 1/4 of your memory.
SO lets say;
you graphics card has 64mb and your have 512 mb normal memory in your pc
1/4 * 512 = 128 mb
so you grahics cards has available: 128mb + 64mb = 192mb
but to be honest if your graphic card has 128 0r 256mb aboard, you can set the aperture size lower then 1/4