I am by no means a computing wonk, but I also suspect that moving from a 3GHz P4 to a 2GHz C2D Core would result in a performance hit for applications using only a single core. My reasoning is that the P4 has a poweful single core architecture optimized for single core applications (it's problem was heat, not speed) and on top of that has a 50% higher clock speed (assuming the above numbers). Besides, C2D Core must have some kind of 'sorter' on the input side (and indeed a 'joiner' on the output side) that splits instructions among the two cores. When one of the cores is inactive, instructions must still pass through it, but it only acts as overhead, as do other processes/controllers/instruction sets that are there for multi-core processing.