So.... Updates on the silent front:
Late last night, about 2 a.m., no noise in the house, no noise outside... Perfect setting for tuning the fans...
First of all, I decided the idle noise of the video card, as little as it was, must be dealt with.
Problem: it won't go under 40%, see GPUZ above, in spite of ultra low GPU temp.
Quick solution: completely remove the Windforce fans.
Long term solution: replace original fans with 2 x 120mm fans. More details later...
Effect of the quick solution: idle/normal use temperature of 35 degrees; tried a quick stress test, stopped it after 80 degrees. Don't know if it can take it, but I want it cooler.
Details on the long term solution: normal width fans (25mm) will barely work. They cross the side of the case by 1-2 mm. Even though it is possible to force the cover on, I won't do it for two reasons: the fan blades will be too close to the cover, thus increasing noise; the cover will push the fans, which in turn will push the video card, which is only a few mm away from the CPU cooler. Solution: use 2 Scythe Slip Stream Slim 120 mm fans.
Now for the nice part: the Gigabyte video card fan connector is a 3 pin type. That is perfect, since it will output variable voltage
Last night I tried with a Nexus and a CoolerMaster, they were barely spinning at the idle setting of 40%. Idle temps around 30; under load, 69 degrees. Hotter than the stock, but still ok and most importantly, current load noise is lower than stock fans idle noise - basically, fan control will go as high as 70-75% under furmark load, much lower (55-60%) when gaming. Now, 70-75% of 12V is about 9V, while 55-60% is about 7V, if that's the way it works. Those are nice speeds to run 120mm fans under GPU load
I will try tonight with the two Scythe fans, temps will probably be a little higher... Will post some pics with these mods and their results.
Now, on the other hand... Removing the video card fans made me notice the CPU fan
Pretty quiet while idle, just a bit over the 7V Nexus fans.
Problem: spin up is pretty aggressive, making the fan noticeable even when there is no need for it.
Solution: remove the red cable from the PWM connector of the CPU fan and connect it to the 5V line of a Molex (tried 7V, still too noisy under load).
Effect: the fan normally spins from 1000 up to 2400 rpm. PWM-ing up to 5V translates into a range of 300 to 1250 rpm, more or less. Even with everything else off and your ear next to the CPU fan, it can only be heard from aprox. 1100 rpms up. Not exceeding 1250 rpms means it is as loud as a 7V Nexus, which pretty much says it all.
The temps? Highest temp during about 1 hour of Prime95 was 64 degrees. Acceptable, I would say. Still gotta see the Furmark plus Prime95 temps, but I have to install the Scythe fans first...
All in all, I am pretty happy with the outcome. I will definitely sacrifice around 10 degrees for a silent computer under normal use, and most importantly close to silent under gaming/load/heavy use.