From my experience it really depends on the PSU you use how far you can push a card and it stays there stable.
I had some rigs with el-cheapo-800w PSU where I could barely overlock a 5870 to 900 MHz and it would get SICK or DEAD every 2nd day, after one of the PSU died i replaced it by a Corsair TX850 and since then I could overclock the cards even to 950 MHz without any problems, they are running for months stable now.
I swapped the second PSU also and hat the same effect..
Dont save at the wrong end
