The way I understand the new cores working. is that say you ran a programme that needed 2.4ghz of processing power and another that needed a further 2.4gz (purely random numbers just for an example)
On the P4 3gz both programmes would have to be ran on one core, slowing the speed each programme works at. Where as on the dual and quad cores, one programme would be assigned to core1 and the other core2 resulting in no performance decrease.