According to the test guys here at Tom's Hardware Sandy Bridge chips start degrading due to electromigration once you start closing in on 1.4v. They have seen some chips start to degrade at a bit less than that as well. For a chip to last and be stable I would keep it under 1.36v unless you have a custom watercooling setup. Once you go over 1.4v chip life can be reduced to a matter of months.
Also unless you have a 3 x GTX580 setup you will see very little gain from overclocking past 4.5Ghz. Some encoding type programs may scale a bit faster at higher speeds but not enough to risk your chip.
Intel offers a special overclockers insurance for K series chips. If you really want to kill one then for $20 for a 2500K and $25 for a 2600K you can get one no questions asked replacement if you kill it overclocking it.http://click.intel.com/tuningplan/