Originally Posted by sebar
Your charts are great but do not tell the whole story. you still need to take into account how much RAM, how many hard drives and system fans you are running. Also you need to consider any expansion cards, optical drives and USB devices in the system. Your first graph shows a system wattage of 470 watts. With a system running at 80% you would need about 600 watts. PSU's run best between around 60 and 85% max load. This chart I put together shows a good range of PSU's to consider taking into account the max system load and PSU wattage.
It has a Corsair AX1200, so under 40% load. The power draw of that setup is measure for the entire system from the wall, so it's probably around 88% efficient. 470*.88= 413w. That's 63% load on a 650w PSU. Extra RAM, HDD, ODDs, etc hardly adds on to that setup, unless you are turning into some kind of storage computer with 20+ HDDs or something.
The whole idea regarding what runs best at what load has to do with efficiency which is an overstated concept. After hitting 20% load, the efficiency doesn't change that much.
Point is, a 650w can safely power a GTX680 in SLI configuration with overclocking.