RetiredChief :
jsc's method is very good and close to what I do. I look for a review of the GPU that I intend on using. Normally Guru3d and anadtech. Guru3d dropped using Furmark to determine there "Loaded" wattage which I prefer. I find that this comes very close to what I measure at the Wall. Then I apply the 60 -> 70 for my Minumium value, which brings me very close to jsc's 50 -> 60 % after factoring in the wall is about 20% higher than PSU load.
I have Not had a PSU degrade to the point of requiring it to be swapped out within a five year window. My concern is not so much the deregulation as it is on the increase in ripple and spikes on the Output - For me, Good old O'scope to verify that.
I do basically the same, with the same results
My main system is similar to thiers, 3.8Ghz I7 / X58 ( had a 965, now a 930 )
Using a Killawatt, I have tested a few cards of varying power on it
GTS450, GTS8800512 + SLI , GTX260 + SLI, GTX460TOP + SLI, GTX570
Have also tested the cards with a variety of Intel chips and chipsets
My numbers for the GTX 570 are close
http://www.guru3d.com/article/geforce-gtx-570-review/8
I idle ~25w lower ( didn't disable power saving features ) and see 371 av. on the KaW when gaming
( have seen higher peaks up to 407, not often and not real worried about a millisecond or two of that )
They use a ~90% efficient CM psu ( 88.6-90.5-88.3 according to 80+ )
I know, "they only test at 23c", rarely passes 22c in my room and my system is on an open bench
Factoring in 89% efficiency = about a 330 watt DC load
Just under 62% DC load on my 520 watt 85% psu, the math pretty much coincides
( I3-2100 and GTX 570 only draw 300 AC running Furmark with same 85% psu, a good 450w would do for normal use )
Nvidia reccomends 550 watts for a system with the 570 ( Manufacturer's add the 38a )
The psu calculator; 548 watts with my input ( no cap aging, 100% + 100% )
As long as one knows to look for a quality psu with a high amount of it's wattage available to the 12v rail either of the above is fine, IMO
If running power viruses one has to expect that they'll see a higher load ( I do it often, worst I've seen with my config. running 10 mins. of OCCT's PSU Test is 530w )
How much headroom one wants based on doing that is up to them, but unless running those viruses 24/7 no more is needed, again IMO
Future upgrades need to be considered, not in the form of " I'm going to add a HD and a few fans", that's pointless