The reason for the chipset change is that Intel's northbridge contains the memory controller which needs to be updated in order recognise both CPU cores. This has caused a lot of people to cry out at Intel's supposed lack of foresight where these people claim that the whole i9xx range of chipsets should have been dual core enabled from the word go. Maybe so, but as the i925X chipsets were being designed Intel was probably still thinking it had some clock overhead in the Prescott which, as it happens has turned out not to be true.