Do they make any difference, like in, gaming or video rendering?
Without that RAM your GPU would be unable to keep up with the demands that the games put on your system. If it can't have it's own buffer separate from the primary RAM then it wouldn't be able to queue items properly for processing.
You see integrated GPUs in motherboards and newer CPUs all the time... these use the main system RAM, though this makes them leagues slower than dedicated video cards with their own RAM. Some video cards have DDR3 and not GDDR5 on them, which is the same as system DDR3, but having it on the same bus as the GPU make it loads quicker. Doing it this way also doesn't eat up your system RAM as integrated GPUs can eat up to 1.5GB of your system RAM.