My card is very stable at 1.21V, 950MHz (1325MHz memory). I originally tried 1.18V, and it was not stable at even 890MHz (playing Crysis 2 would cause the driver to crash repeatedly). At 1.21V, 950MHz, Crysis2 (as well as Kombustor) is 100% stable. With custom fan control, the card peaks at about 76*C, 75% fan. (Card idles at 38*C, 10% fan.) I am VERY happy with this card.
In my opinion, modifying the BIOS is just plain stupid. All benchmarks have shown that unlocking the additional shaders (ie, the 6950 to 6970 hack) only provides a very small (< 5%) performance boost, at the cost of stability. On the other hand, overclocking the 6950 DCII provides a much larger performance boost (15-20%).
In regards to voltage modification, it is not possible to use MSI Afterburner to change the DCII's voltage. The DCII uses a different voltage control chip than the reference card, so the only way to change the voltage is with Asus SmartDoctor. After changing the voltage, other utilities such as GPU-Z and Afterburner still report stock voltage (1.1V), but the card *is* in fact using the voltage set by SmartDoctor, as can be confirmed by much higher temperatures when benchmarking.
One problem I do have though is that SmartDoctor does NOT retain the voltage setting after a cold boot, it must be manually set at each boot. Also, after setting the voltage with SmartDoctor, it is wise to CLOSE that app, as when the app is left running weird things tend to happen (perhaps it is conflicting with MSI Afterburner somehow.)
All in all though, I love this card.