the 1.5 processor will run just as fast if not faster then the P4 3.0. why its becase centrino processors have bigger caches and so they can instantly compute information faster. it the same idea(i think) as a mac G4 1.7 being as fast as the P4 3.5 for something along those lines.
that is also part of the reason centrinos saves batt life, its because it uses less power to do the same task as a P4, and it needs less cooling.