Don't worry about the wattage. You're way in the comfort zone, really.
I have a similar rig, the i7 2600K, 6950 2gb (Twin Frozr III edition, slightly OC'd), 8gb of 1600mhz RAM, and I also have 4x 7200RPM harddrives and an SSD. I'm running all of this with a 650w PSU, and I've never had a problem with it at all.
My guess is that there's something wrong with your video card. Try reseating it, if that doesn't help, I would RMA it.
Also, like SS said, don't ever think of overclocking your video card to solve a problem like that. That won't work.