Basically, you double it on the 64bit chips, give or take off about 350-450 MHz on standard Athlon64chips, and about 250-150Mhz on FX chips.
The math is like this.
AMD Athlon64 speed x 1.656= P4 speed
AMD Athlon FX series Speed x 1.849=P4 speed
Turn the equation around to get the P4 speed. Also take into account the fact that AMD64 uses a 1600MHz Front Side Bus as well, so this plays effect as well.