I once had a video card fry itself due to heat, and that was without overclocking it.  My 6800GT would frequently hit well over 100 degrees C (according to the software that reported the temperature; I would not have believed it possible for IC chips to continue working at all at such a temperature).  Eventually it got to the point where my system would not run for more than a minute or two after booting up without crashing (in this case, displaying a constant pattern of random colors on the screen and do nothing else).

I replaced it with a 7900GT which runs much cooler.

Anyway, in most cases I don't recommend overclocking, or at least be very careful with it.  In MikeJ's case it seems like the OpenGL functionality on his card can't handle the increased clock speed.  (note that I have absolutely no idea as to the internals of the card itself).  If Poser 7 works fine at your card's stock speed but crashes or hangs when you try to overclock it, then you shouldn't overclock it.