3.5Ghz is about the limit for most Northwood P4s, its not very practical to push much further. Some special batch Northwood P4s can reach 3.6-3.8GHz, notably the so-called '30-cap' units. But honestly, the age of P4 is long over. Large majority of today's processors can easily beat a P4, even single cores, and they take up less than half the power required by the P4 to do the same amount of work.