More info would still be nice, but lets go worst case: Assuming you are already running the computer 24/7, assuming it is an ATI 6970 (I will assume you are going high end), the GPU will be using just over 200 watts at maximum draw. The baseline draw of a 6970 is about 70 watts, so that would be a 130 watt increase. Basically, the same amount of power as 2 normal light bulbs, or a 32" LCD TV.