EvilMuffin253 :
You seem to miss the point...
even if you add a ton of peripherals, the one thing that causes a strain on a psu is another gpu. if 2 gpus can easily run on 750w psu, then its extremely easy for a computer wit 1 gpu to have alot of additional add ons(tons of HDD's case fans etc) on a 650w psu and still have room to overclock. if the point i missed was a psu. although the 570 stays on budget, there is absolutely no reason to run a 6970 at all, its more expensive than the 7950, less efficient, and is an inch longer, which brings up the case issue again.
tl;dr, the 7950 choice is overall better size and power consumption wise, psu is over necessary even if you add a bunch of addons onto the mobo(and the oem mobo probably lacks alot of addon slots as well)