for a extremely long term oc, I always use 10% as a general rule, which 1.27*1.1=1.397v. +10% is also the limit of what you should do with air cooling keeping safe. Look at peoples temps with higher then 1.4v on air without living in alaska. Almost all electronics parts permit +-10% so thats as far as 'safe' is. With water you should be ok to 15% or 1.4605 but i wouldn't go past 20% or, 1.524v (sounds familiar?) if you want your cpu to live anywhere near a long time, but 20% is very much so pushing it
Reason why you get higher limit with water is the cpus were designed with heat transfer coefficients that air cooling offers, water has higher transfer coefficient due to heat capacity, and surface area advantages.
So unless you are awfully unlucky, 10% with air, 15% with water and you should have a cpu that lasts years so lets stop talking about voltages now
Temps are extremely important too, voltage tolerance goes down with higher temps, Dothan was a fine example of this if you guys ever played with doing strong dothan oc's. you started needing to go do chilled water at about 3 ghz because the temperature tolerance was way down due to the voltage, not because it was putting out a lot of heat.Warning: Spoiler! (Click to show)
Edited by ryuji - 2/24/12 at 10:53am