its called overclocking
except, the method you propose has some negative aspects. with regular "modern" overclocking, you set the PLL to multiply the base frequency differently. So, you see a higher MHz listed... the computer "knows" it's running faster.
Your method would not be detectable by the computer itself. it would report standard speed, but would run faster, and the clock would be faster, etc. It would probably cause trouble if you have cards that have their own crystals... like network/video cards. either way, you might get network problems since stuff needs to be sync'ed.