What exactly do you want to do? The amount of memory a given screen image will take on the video memory can be calculated as
Horizontal Res * Vertical Res * Bits per pixel /8/1048576 = Mb
However, many video chipsets these days store images offscreen to make updates faster, and movement of 3D images (such as in games or screensavers) will need more memory, dependent upon the actual application.
John