I solved the issue (following a Phoronix forum suggestion to try and search for an updated GPU BIOS )
I'll write the complete answer so people facing the same issue might stumble upon this solution.
In order to find which BIOS to flash, I first went looking for the current GPU BIOS version at:
$ cat /proc/driver/nvidia/gpus/0/information | grep BIOS
Video BIOS: ??.??.??.??
In short, the nVidia driver, beyond not being able to detect the fan speed, was also unable to detect the BIOS version. In short, something is very wrong the machine's POST sequence.
Went into the Intel MB settings (BIOS) and under Advanced -> PCI Configuration was Legacy VGA Socket which was configured to initialize the wrong slot (Slot 1 instead of Slot 2).
Setting it to Slot 2 + complete power off + PSU disconnect (a simple reboot didn't help) and I've got a silent machine.
Most likely this issue will prevent me from installing a second GPU, but to be honest, at least for now, I couldn't care less
 Credit: http://phoronix.com/forums/showthrea...266#post381266
DEV-NG: Intel S2600C0, 2xE52658V2, 32GB, 4x2TB, GTX680, F19/x86_64, Dell U2711.
DEV: Intel S5520SC, 2xX5680, 36GB, 5x320GB, GTX550, F19/x86_64, Dell U2711 (^).
SRV: Tyan Tempest i5400XT, 2xE5335, 8GB, 4x2TB, 9800GTX, F19/x86-64, Dell U2412.
LAP: ASUS N56VJ, i7-3630QM, 16GB, 1TB, 635M, F19/x86_64.