I suggest a Google search for how to overclock would be a good start.
I must stress this: do not attempt overclocking unless you are 110% sure that you know exactly what you are doing, and have the expertise needed. You can and will damage or destroy expensive components if you are not extremely careful.
As for the limit, it depends on the individual chip and environment - no one can say exactly what the limit of any given CPU is for overclocking as they are all slightly different, and the environment and hardware they are used in or with are also different.
