The only thing that can have an impact on CPU lifespan is high voltage. As long as you are not raising it to extremely high levels, you are not likely to reduce the lifespan of the CPU by much, if at all. All you are doing is raising the clock speed as high as it will go and run stable. The only reason the manufacturers of CPUs do not do so is that they want a little breathing room as far as being certain that it will run at the speed they rate it at. If it runs stable at a certain speed, there is no reason to think you are shortening its lifespan. I mean, some chips overclock well, while others do not. Some chips should have been rated at a higher clockspeed in the first place, but end up being downclocked due to a high demand for the lower speed chips. Those are the ones you want to look for to overclock, since they are cheap and can run comfortably at a higher clockspeed, and usually with little or no voltage adjustment. The XP1700+ was a prime example of a chip rated at a far lower speed than it is capable of running due to the demand for it.