You are right in that pushing too much voltage into anything over what it's designed to handle will decrease its lifespan, though I've never seen evidence of this first hand. I had an AMD 2.0GHz chip overclocked to 2.7GHz for over two years, and despite the extra voltage, it ran fine.
I have no theories to back this up, but I consider 1.4v to be the maximum safe voltage. Normal operating voltages are between 0.85v - 1.3625v, so I don't consider 1.40v to be risky business.
That said, anything over 1.4v, I'd personally stay away from unless simply trying to achieve a large overclock. My goal in these reviews is to stay as close to 1.4v and under as I can, and in this particular review, I didn't have to go over.
The Q9450 review I'm working on is a bit different though. Quad-Cores are harder to overclock, so they need more juice, so I have it at 1.475v at the moment, and might go over 1.50v for the simple sake of finding a max overclock.
In your E8400's case, don't worry about going up to 1.3625v, as that's within the CPU's spec and would be covered by warranty if something were to ever happen to it. But again, I've never heard of a story where a CPU has died early because of too much voltage (unless it was really extreme and the people overclocking it were just idiots or were trying to kill it on purpose).
