as long as it stays at 42c it will last a long time, dont forget that chips are supposed to last years at around probably 60c (well, I looked at MBM5 on my work machine, a celery, something i am not supposed to, and that is at a steady 63c, and that is due to last for another five years)
My second rig has a T-Bird 1.4 sitting in it running at 1500MHz at a temp of 47c, and this is fine. The easiest way to tell if the overclock is starting to kill the machine (and you will see this happen from the out set of the overclock) is by a lot of crashing, if its stable, then it will last until you decide to upgrade anyway, and it will still keep going!