Great guide, but does it take all things into consideration:
Cards take 75W from the PCIe slot and the rest from 6+6 pin cable on 12V rail (e.g. in case of nvidia GTX260).
Great number of PSU provide certain number (from 2-6) 12V rails of 17-20A, only few of them have one 12V rail with "huge" amount of Ampers on your disposal.
So, eg. in case of 580W PSU with 2 12V rails of 17A you should have on your disposal some 408W (the remaining power goes to -12V, 3.3V and 5V rails)... anyway, that should be enough for GTX260+nVidia MBO (from chapter 4 -> 150W + 232W) and the PSU should work in its "optimal" load.
But does it? I am just theorizing, what is the power distribution in that particular case? How much Amps is already spent on one 12V rail from MBO/CPU or is it distributed to both rails equally?
And if we connect mentioned GPU can it easily overload one of the rails?
I am very much against buying overpowered PSU in any case and would always prefer using the optimal one (minimal offcourse

).
I would like to hear what you guys think about this.