Initially, I was going to say that, unless you are planning to overclick or to add video cards in SLI, that you could do just as well with a 650W power supply. However, the values I have been getting from various power supply calculators seem to vary wildly. (Some of that may well be due to particular calculators using out-of-date data. Also, we can't tell how many USB devices you might use.) As such, lowering the p/s wattage may be good, but don't go too far. 650W-700W SHOULD be enough, but I would suggest doing your homework for it.
Finally, for a gaming system, I found it odd that you omitted getting an optical drive, either a Blu-ray reader or a DVD burner.
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