Originally Posted by TDS
You will be more than good. Check my sig rig out.. I am running a HX 850... and with a kill a watt meter.. I pull max 520watts
I would suggest the Corsair over the XFX one you listed though.. at least in my research when I recently upgraded for Xfire.. I found the XFX to be lacking in connections.
When talking about power draw at the wall (when talking about using a meter like the Kill A Watt), we're talking about a power draw that's higher than the power that the system is pulling from the power supply because the PSU is not 100% efficient.
So let's pretend that the efficiency at the time of this 520W power draw at the wall was 85%. If that were the case, then the system was pulling 442W from the PSU which means his PSU (the HX850) has more than twice the power capacity he needs for this particular power draw.
To clarify it further, let's say the efficiency was 80%. If it were, then that would mean the system was pulling 416W from the power supply at that time.
Now, AMD recommends a 550W peak-rated
PSU to power a system with a single 6970 in it (proof
). Now, most of these pages on AMD's site that show this also show the recommendation for the same card in Crossfire. This one doesn't, but every page I have ever seen where the Crossfire recommendation is mentioned, it's always exactly 100W more for the Crossfire recommendation. So this means that I believe AMD would recommend a 650W peak-rated PSU to power a system with two 6970s in Crossfire.
So this would mean that a 650W continuous-rated PSU (such as the HX650) would be more than enough, and I think TDS proved it with his 520W power draw at the wall.
In other words, save some money and get a good 650W or perhaps a good 750W at the most if you don't believe us and need peace of mind. But a quality 850W would be unnecessary extreme overkill and it would even result in a lower efficiency due to the smaller power draw on such a PSU as compared to that of a quality 550-750W. Generally speaking (very generally, here), PSUs are at their most efficient when under a 50% power draw. So since we aren't putting 50% load on our PSUs at all times, it's best to buy a PSU where the load on the PSU while gaming is at least 60-75% or perhaps even more. That way, the average efficiency is maximized.
Here's more information on efficiency:
On PSU Efficiency
PSU "50% Load" Myth