I have a GTX 465 that unlocked to a 470. I have run the volts up to 1.215v on a few suicide runs (950 / 1900 / 1900 clocks) with the original cooler @ 100% fan. It would be stable with that setup IF I had water cooling, Im not comfortable with 95c temps in furmark. Temps usually were around 80c in games at those settings, but for safety I dropped back down to 800 / 1600 / 1800 @ 1.087v.
In reality, for my setup the conservative settings are better. I can run almost everything with 8-32x FSAA and max details, but I am resolution limited at 1680x1050.
For your problem, a modified vga bios is REQUIRED for more than 1.087v. You didn't mention anything about flashing it, so I'm going to point my finger at the NV voltage limitation in the BIOS.