There was an article at Anandtech, Debunking Power Supply Myths, and this page (bottom) may have the information you want:http://www.anandtech.com/casecoolingpsu ... i=3413&p=6
Wow, that's exactly what I needed to see. Thanks!
According to the graph, the noise threshold will be the same up until about 250W on both Modu82+ PSUs. However:
The Modu82+ 525W fan starts to ramp up around 250W.
The Modu82+ 625W fan starts to ramp up around 310W.
For me, it's probably worth the price premium of the 625W. I'm currently running a Seasonic S12+ with a slightly noisy fan, i.e - slight clicking noises when my system isn't under load. I have to RMA it but will likely give it to the wife.
Anyhow, the Modu82+ 625W would give me more leeway for power hungry passively cooled videocards, such as a 9600GT. As I'm running my E8400 Prime95 stable @ 4Ghz (with a Zalman CPNS9500 < 5v!) but I'm currently limiting myself to a HD2400 due to the low power requirements. This keeps me under 250W on my current PSU, which also increases in noise past that..