Overclocking is a hot topic in most PC Magazine.
The motherboards, CPU manufacturers all said that it can be overclocked and remain "stable".

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That is correct if you know what you are doing, you obviously don't.......

Overclocking is fun, expensive and you can learn a lot - but it's not useful, that's the point.

I remember some time ago, I had bought a new Radeon graphics card for 300EUR and immediately wanted to overclock, just for fun.
As a result, the complete cooling unit had molten off (plastic clips) and dropped on my hauppauge, which was placed underneath. The experiment left me with a dead graphics card and a dead hauppauge. But I had learned a lot ...

Overclocking is a hot topic? You are funny.

I used to overclock my computers. I found that it was not worth the trouble. 10% increase in speed is not worth the effort.

If you researched the history of overclocking you would find that the idea started because CPU manufacturers used to sell fast CPUs as slower models in order to fill their orders for the slower models. Customers discovered this and found that some CPUs marked as slow would run much faster than they were supposed to run. But this did not apply to every CPU. It was like a lottery.

In order to overclock most CPUs you need to add special cooling. Water based coolers or Peltier coolers are required to offset the higher heat generated by overclocking a typical CPU.
http://en.wikipedia.org/wiki/Thermoelectric_cooling
Also, you generally have to increase the voltage to the CPU in order to make it run stable at higher frequencies.

As others have already said, if you don't have the money to replace the CPU then don't do this.

how about to Unlock the Cores of quad or hexa cored CPU?
why need to unlock?
and new thing like Turbo boost, look very fanciful.

Why do you have to unlock a CPU in order to overclock it? Just think about it for a minute from the manufacturer's point of view. If you are selling several models of a product at different prices and if people discover that the lower priced product might work as well as the expensive model then you would want to prevent the lower priced product from working as well as the higher priced product. Since the lower priced product and the higher priced product are sometimes the same product then you have to add some performance limiter to the lower priced product. That is the "lock" on the multiplier.

I don't know why you think Turbo Boost looks fanciful.
http://www.intel.com/technology/turboboost/
It sounds to me like the CPU normally runs below its rated speed until "Turbo Boost" is requested.

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CPUZ shows me that my bus speed is the one I fixed in BIOS, but the mutiplier is ever changing, when u run more programs, it will increase, when u r idle, it will reduce and clock speed decreases.

the Windows OS is smart, it control the clock speed to its optimal level.

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So from a sample set of one you have determined that everyone else is wrong. Your own question says that your conclusion is wrong. Your CPU is running hot. Heat can damage the CPU. Your own experience says that your conclusion is sometimes wrong. It depends on the circumstances (CPU, percent of speed increase, required voltage increase to achieve stability, et. al.)

Overclocking by varying amounts introduces proportional risk of damage to the CPU.

As I said before, increasing the CPU speed by 10% is negligible. Overclocking is, IMO of course, only worth doing if you can increase the CPU speed significantly, such as by 50%.

Why do you ask questions if you think you already know the answers? Of course the motherboard clock speed remains constant. All of the other devices depend on a constant clock speed. Given that then the easiest way to change the CPU frequency is to change the multiplier on CPUs that support that.

Windows and Linux can dynamically change the CPU frequency. So what? Windows is "smart" in this way. So is Linux. So what?

One minute you are talking about one thing. The next minute you are talking about something else. You shouldn't change the subject in the middle of a conversation. You were talking about overclocking. Then you add a comment about dynamically changing the CPU frequency as if it has something to do with overclocking. It doesn't. It is a different subject.

Did the motherboard manufacturer give you a guarantee that your processor wouldn't be damaged by using their overclocking facilities? (Given that I have never seen that, I'd guess no.)

I've never seen a manufacturer of CPUs guarantee that their CPUs would overclock (likely to, maybe, guaranteed to, no) and the best that you'll get is 'it may overclock, but if you overclock it and damage it, then it is entirely your problem'.

So, why are you even considering overclocking? Given your reaction to the temperatures, you don't know enough about it, you know that there is an increased chance of CPU damage and yet you state that you can't afford it to go wrong, because you can't afford to replace the parts. So, why increase the possibility of failure?

Just don't do it. It doesn't make sense for you.

now my CPU is cooler, previously i did not why it was high,
anyway, i just overclock from 2500 MHz to 2640 MHz,
yeah, not significant, but no harm to do that.

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the Bios menu has CPU Health Status, it shows my CPU temp is only 42-50 oC fluctuating,
when i run OS it should shoot up higher, but i am doubful it can be 78oC.
right now Everest shows the temp is about 65-68oC.

Get it hot enough, and you can have a barbecue.