I can't really tell you how to do it, as it depends mainly on your BIOS. I would suggest that you check and see if there is a BIOS upgrade available for your hardware. Sometimes a later BIOS will have more features to choose from. You should be careful when pushing the envelope however.
It might be nice to know how they get the various speeds of chips from the same architecture. The chips, or dice, (singular is "die") are made on a wafer, usually a round substrate that is about 150 mm in diameter. Wafers are not without flaws. The dice are tested on the wafer and sorted by the amount of current drawn at given test frequencies and voltages. Dice that have more flaws and distributed capacitance than others draw more current and are rated or limited to lower frequencies. This is, very briefly, how they get the different speed ratings out of the same architecture.
When you over clock, your chip needs to draw more current to keep the signal voltage levels up to the threshold minimums. Otherwise the chip becomes unstable and won't run correctly. To do this you must increase the voltage going to the chip.
On a very basic level, Power=Current X Voltage, (P=IE). Since the current and voltage have gone up with over clocking, you can see that a considerable power increase has occurred. It being the product of the two variables. Power is dissipated in the form of heat. There are limits to what temperatures can be tolerated by the chip in an attempt to dissipate the increased power consumed.
In practice, some chips might be on the border of a cut-off point, either on the high side or the down side. Some might be actually marked wrong. People actually do this task, so mistakes are not unheard of. It is a crap-shoot as to what you actually have. That is why nobody can tell you how high you can go with a given chip and hardware set-up.
I probably told you more than you really wanted to know, but it is what it is.