I'm looking at building a custom laptop I know the 580m is a bit better than the 6990m but it is much more expensive to put it in a sager. Is the price difference with it to get the 580m? I can get a pretty kick ass laptop with a 6990m for a little over 1600. While the 580m takes me to almost 1900. I'm sure out will come down to preference but how different are these cards in terms of gaming performance?