On an average system there is recomended that you use a 700w minimum with dual 670's. And, if you overclock gpu's, cpu and have a lot of fans/hdd's and a few pumps, thats gonna need power too. Using a 850w for this system might be very vise and also future proof in case you wanna use/do all the things as said above. 850w way overkill? no, not really.
They normally go for around 140-145 anyway, so that's about right.
I got mine for £139 new plus postage from ebuyer (free delivery but thats not the point)