IIRC with a graphics processor, it's best to have one that doesn't pinch memory off the machine. Now I may have that backarsewards but I'm sure I've read somewhere that there is a best configuration for gaming gpu's. Best thing is to read up on gamer forums or even graphics forums. Nvidia's website certainly has user forums.
Spanners prolly has a point that a games machine is best for gaming unless of course your son needs to use a pc for work too.