I want your GTX460 vBIOSes!

[Ket]

Does the flashing procedure in the OP flash both cards in an SLI setup, or just the top one?

Top card only. You must tell nvflash which card to flash otherwise it will just flash the primary card.

No. GF104 GPUs have a hardwired VID, which varies from 0.937v to 1.0??v.

No, they don't. You can set GPU voltage anywhere between 0.82v and 1.2v via vBIOS mod.

Thanks a lot for the advice!
And OMG, what a difference....
I had to go as low as 803 core to become error-free with stock voltage...
And I could pass 4 hours furmark with 846 core.... isn't that extreme?

Anyway, since I am not going for extreme fps, but want 100% stability, this error check ability of OCCT is great. I will reach a point where I am error-free for hours and leave it there....

Furmark/Kombustor/OCCT are absolutely useless. In the case of Furmark it just puts a lot of stress on GPU and VRMs, Kombustor is just crap, never seen it take the GPU over 15% utilisation. OCCT is just a torture test, not a stability test. If you want to truely test your graphics card properly for stability (GPU and mem) use Unigine Heaven 2.1. Set max AA/AF and resolution and just let it loop. GPU stays over 90% usage basically always and Heaven gives the memory a proper workout so you can test for artifacts at the same time. Kill two birds with one stone, if you will.