my goal when i overclock is to find a nice balance between temperature and voltage. voltage will raise temperature a bunch, but voltage will increase stability. in short, the better your cooling device the more you will be able to overclock.
of course more heat = less lifetime for the processor.
also, what socket? the 754 version is 130nm putting out alot of heat, if it were 939 then it would be much more overclockable.
also, note that when u overclock your processor, it also overclocks your ram. this could be another reason it is freezing.
is your video card overclocked? that could also be another reason.