I'm perplexed. The chart says GPU usage with a 460 is only 141W. A 2500k will only use about 80W max. Adding another 460 will equal 362W. Adding motherboard and peripheral power usage will add up to 450W max.
Why would you need 750W or even a 600W PSU? With the trend for energy efficiency through smaller fabrication, power usage should drop in the medium term for both CPUs and GPUs.
(I'm saying this in part due to not wanting to spend so much on a new PSU myself and making the case for getting a sub 500W PSU). You could save money and upgrade another component of your system and still have a good PSU.
Because the power you are talking about needs to be supplied on the 12v rails not the total power.
If you are talking about needing 450w + 150w on the 3.3v and 5v thats a minimum of 600w to start with.
Also if you look around half the time the more powerfull units are the same if not cheaper than the lower powered ones so it makes sense to buy the more powerfull units and give yourself some breathing room.