It's definately not enough for 4870x2 plus an oced 65nm quad. It'll boot, but crush on full load. Brands aside, a tractor made by BMW is still just a tractor. When it's not enough, it's not enough. You'd need at least 750w with good 12v rail ratings, ideally 850+w just to be safe and leave some headroom.
WTF? since when is a corsair 650 a "tractor" ?
Go ahead and run your 850w psu and waste all the electricity you want. I hear energy is cheap right now.