I was farming with a single XFX 6950 (flashed shaders, 1145mV, 940mhz core/ 625mhz memory ~ 405mh/s) for a while now, and got another one today.
Curiously, when I stuck it in, overclocked it with MSI Afterburner, they make the entire system hang in around 3 minutes. At first its the UI, then the mouse freezes as well.
Now I'm running them both at 1145mV, 920mhz/625mhz at around 390mh/s or at a total of 780mh/s and it seems stable.
Should I up the voltage, or the memory clock, or something for them to be stable at 940mhz core? I am not sure since if it would lack juice it would hang immediately, not after 3-5min of hashing. The temperatures don't go above 75'C.
The power supply is a Corsair TX750 V2 and I'm using GUIMiner.