I can only assume that you have an Intel. First of all if you are going to go into you need to get an i875 motherboard. It is the best that they offer and should give you no problems. Anything lower can have weird problems with hardware. Anyway if you get an Intel motherboard it will not let you over clock. No matter what other hardware that you have in the case. They use this as a mechanism so that it will not wear out the hardware faster. If you want to overclock you should go with AMD's motherboards, but overclocking is really overrated anyway. Just stick with the Intel and don't overclock. It's really quite useless.
No they don't let you overclock. In fact if you somehow do find a way around their preventative mechanisms and the motherboard has a problem, it voids the warranty. Besides overclocking really dosen't add that much speed at all to the speed of the computer.
Overclocking was meant to increase the speed of older and slower processor for games that requires a bit beyond what the processor offers. Other people overclock their processor to gain extra numbers in generic benchmarks. Overclocking a fast enough processor (i.e. 2.8c) will not give you any noticeable difference in terms of performance in todays applications.