Burebista is 100% correct. Intel processors are very well designed and there's no need to worry about the core temperature as long as your computer is running stable.
HTC: TJMax is definitely not off on my chip. My reported core temperatures on this CPU are accurate to +/- 1C when these sensors are in the range of 70C to 100C and I'd be very surprised if your sensors weren't just as accurate. Most of the problems with the sensors are at idle but when the temperature gets up into this range, they are very accurate.
I have a Tuniq Tower which isn't a top rated heatsink anymore. I also have the fan on the lowest setting because I hate noise so that's why it was easy to get the cores up to 76C in a hurry.
The bottom line is that if you run a 45nm Core 2 Duo at 4GHz and you need 1.40 volts to be stable, you're going to need a top rated cooler and some good airflow through that cooler to keep the core temperature at a reasonable level so you will be able to maintain stability.
If you go for a more modest overclock like 3600 MHz, I found that my CPU is stable enough that it can run at full load for hours even with the heatsink fan turned off. Does the core temperature get hot? Of course it does. During my test it was bouncing off the thermal throttle for 3 hours straight while continuing to run Prime95 Small FFTs just fine.
This exact same CPU is capable of running reliably at 4GHz but it's not capable of running at full speed and at full temperature. The only way to run at that speed reliably is if I keep the core temperature at a more reasonable level like 70C.
The exact temperature number is completely unimportant. The only thing that is important is that whatever overclock you choose, you're able to run at full load for an extended period of time without losing stability. If your core temperature is too hot, Prime95 will either return an error or my motherboard / CPU would typically reboot when pushed too hard. Ignore the temperature number because it's just a number and concentrate on stability. As long as your computer is stable, the exact temperature your CPU is running at is not important. The cooler you can run your CPU the better because it will give you more overclocking headroom but other than that, no need to be overly concerned about your core temperature.