The recommended limits for a processor manufactured by Intel on the Prescott/Prescott 2M core are as folllows (please note that these are recommendations and may not reflect on the actual limit):
Maximum recommended temperature: ~66C
Maximum recommended voltage: ~1.525v (may increase depending on cooling method/temperatures).
These limits should be followed no matter what clock speed the CPU operates on (this is arguable depending on the situation), you may have a 6GHz Pentium 4 and a 3GHz Pentium 4 still they should both be kept below 66C for best results and for safety reasons.
Going over this limit will do little harm to a degree.
Intel's TM1 Technology (Thermal Throttling) is there to attempt to cool down a processor when it reaches what it believes is the current CPU's Thermal limit. If this happens it tells you that your CPU is running to hot. This does not work without the amount of voltage applied, so you have to limit yourself on the amount you use. The motherboard will limit you also to a degree to prevent very high voltages from being used that could cause harm to both the CPU and the motherboard.
When a higher voltage is used it is always best to keep the temperature as low as you can, the TM1 state will not change however, but you should try and keep it cooler. Once the voltage hits a certain point there can be something called quantum tunelling (electron transfer to other transistors). Even with adequate cooling this can happen at high voltages, this is a good reason to keep your CPU at a decent operating temperature.
If the guidelines are met or kept below them there should be no problem
Anyone want to make a point about this post then please feel free