I'd bet that during testing they realized that the speed of the ram in the video card counts a whole lot more than the amount. Crank your current video settings up to 1600x1200x32 bit color and see how much video ram that actually takes. Multiply that by 2 and that is really the maximum amount of video memory you need.
After that, I'd say the rest would just be used for caching, and given the cost of high-end ram and the price points they can sell video cards at... well, you get the point.
EagleDNY
$.02