why 850W ?

1 3870x2 is like 200W max
1 Q6600 is NEVER above 200W, more like 100-150W
Mainboard+other stuff < 100W

if you count that together you get 700W max, but probably even 600W would fit.

ok, add some more for safety, but 850W is not necessary.

Of course it all depends on 12V lane(s), but that's another point.