I can't praise this build enough, but one of the author's comments drew my attention:
"It is crucial to choose a relatively high-wattage and an 80-plus efficient power supply. The high wattage is going to provide ample, clean power to all of the components with a great deal of headroom. Such a power supply is extremely unlikely to overheat and will thus operate at a relatively low temperature. Consequently, the cooling fan is not going to have to spin very fast, and the power supply will remain quiet, even at full load."
The conventional wisdom is to choose a lower-wattage PSU. For an office-use PC, even a 200 watts PSU suffices if using integrated graphics (but then, try to find an 80 plus PSU under 300 watts).
In this build, 500 watts at most should be enough, or not?
Does it make sense to select a higher-wattage PSU for the sake of silence, as the author says?
It's not the only way to get a near silent PSU, but it is an effective way of doing so. I chose the CP-850 for my rig for the same reason -- the massive amount of headroom ensures that the fan won't spin up until a high load (400~500W in the case of the CP-850). Make sure that your rig doesn't exceed this threshold, and that the PSU always gets fresh cool air through a dedicated intake vent, and the PSU fan will only ever spin at its lowest speed.
The other options would be a low wattage fanless model, or a low wattage fanned model -- with this latter option you would need to research the fan speed curve and compare it to the power draw of your system. See the second-last table on this page
, where the fan in the Seasonic G360 approaches 20dbA at 200~250W, compared to 400~500W with the CP-850.