They are seeing 433 Watts of system power consumption with 2 cards in SLI running at max load.
You could have gotten away with a 750 Watt, but 850 isn't outrageous. I would keep the power supply that you've already got being shipped to you. You want to keep it at around half load for maximum efficiency.
Nothing will happen. You will just have a low load on your PSU. Like geofelt said, it will only deliver the power that your system requires. The benefit to getting a 750 Watt PSU is that you'll probably be putting around a 50 or 60% load on it, and that's usually the range where your PSU is most efficient.
But yeah, nothing bad will happen if you use an 850 Watt.
so i should buy a 750w like I had bought instead of a 850w ?
what can happen if i run my pc with one single gtx 670 and 850w psu for long time ?
im a little bit confused
A psu will operate most efficiently in the middle third of it's range.
It will also run quieter if it does not need to run at max.
Overprovisioning a bit is a good thing.
If anything, it will prolong the life of your psu.