I'm wondering what you're doing with the memory speed on each setup. If the 3.6 is dropping the memory speed, that may explain the slower benchmarks versus your 3.4 at stock if the memory is running faster there.
Also, regarding the limited overclock on your 3.4 chip, that also may be a memory issue limiting your overclock. Try slowing the RAM down some to see if you can reach a higher overclock.
You really shouldn't be able to hurt these Prescott chips with overheating since they all have built in thermal throttling that will shut the chips down if they get too hot. Now if the guy ran too much voltage through the chip, then yes, he could have damaged it, particularly if he ran it over-voltage for a long time.
With over-volting you speed up a phenomena called "Electron Migration" We first saw this in the Northwood P4 chips and it was called "Sudden Northwood Death Syndrome". Some people found they suddenly could not even run their chips at standard speed without instability even though they had been running at successful overclocks for some time.
I haven't heard much about this with Prescott chips however, I think because most people tend not to add much voltage to these chips. Adding voltage to a Prescott dramatically increases heat which often puts you over the thermal throttling limit, the chip then slows itself down and your overclock is worthless.
Its definately not your mobo. Did you ask your friend from who you got the chip if he oc'ed it? If he changed the voltage? Why was he selling the chip? Just maybe there was something wrong with it or there still might be but thats only the worst case scenario. Just find out and let us know.