my 2 cents worth is that you should be fine with any voltage acceptable to the Rev E S939 processors (in other words all the recent cores; Venice, San Diego, Manchester, Toledo). so up to ~1.6v should be ok with good air cooling or passable water cooling. however for the newer 65nm cores i can't be sure what the safe limit would be, though i can say with certainty it would be lower as is always the case with a lower die process size. my guesstimate would be ~1.55v as a comfortable medium.