Those are obviously connected issues. You overlooked voltage, which is the real cause of the damage. The way the CPU works, it jumps to a higher voltage to make sure that the highest CPU setting is stable, and the setting for the highest voltage on the N900 is already "overclocked" in the sense that it is known to reduce operating life.

What should be discussed in this thread is how to reduce the operating voltages and run at the same (or maybe slightly higher) clocks. That would increase battery life and not adversely affect phone life.

Everything is connected, so running at full CPU speed for long periods of time, which will heat it up, will accelerate damage to it. How much and how fast it heats up obviously depends on the maximum speed you have set, and the voltage it operates at for that speed--and your particular CPU.