I think you are referring to the AGP aperture with the default for that generally being 64mb. Often with old AGP cards 256mb or larger you would increase the aperture to see some additional system memory shared with the video card.
A frame buffer is the portion of the video memory that holds the pixels used for the refresh rate of the display there. This is mainly seen as a device that provides 2D, 3D accelaration. Increasing the aperture was one thing done by gamers on the older systems. A good description for that is seen at AGP Aperture