You can't use your system memory for video ram, the hardware simply is not designed to do that.
You're probably just working with 2D applications I'm assuming. Are you able to get the resolution and color depth you want? If so, there's no need to upgrade to a video card with more ram. With 2D applications, to display a certain resultion and color depth, it takes a fixed amount of memory - in other words, don't upgrade thinking you'll get a faster system.