I have a 450W PSU and my fan runs with maybe 300rpm and is absolutely unhearable inside the case (even when it's night). So i won't count this as an argument.

~ 230W = 30 % Load (760W) ~ 135W = 30 % Load (450W)

The idle draw of such a system is below 100W (okok.. max 100W ;) ) so you won't hear any fan even with a smaller PSU.