The dual core and quad core processors have become very popular nowadays but the main reason behind their success is the power management.
There are two basic concepts that one must know, they are :-
Overclocking the processor :-
Let's say that ideally a normal single core processor takes x units of power to give x units of performance. But users generally tend to increase the performance by overclocking the processor ie essentially increasing the clock frequency of the processor. If a user follows the process of overclocking the processor, then the performance increase is about 12-13 percent but the power increase required for that would be about 70 percent.
Under-clocking the processor :-
Now the interesting concept is what if we under-clock the processor that is decrease the clock frequency of the processor?
In this case we could get about 80 percent of the performance at roughly half the power. For example we would get .8x units of performance at .5x units of power.
Now this is the concept that is utilized by the dual core processors and instead of using a single processor we combine the two processors and operate each one of them at half of it's power and so for the same power taken by the normal single core processor we get about 70 percent increase in performance. The aforementioned reason is the core reason behind the success of dual core processors.
Another major factor in dual core processors is as they operate at lower clock rates it helps in reducing the speed mismatch between the processor and the memory and this is a relevant factor because with processor clock rates increasing at very faster speeds the corresponding increase in clock rates of cache memory is not their, therefore a processor's clock cycles are not utilized properly and are wasted in memory read/write cycles.
Hence the concept of under-clocking helps also in reducing the speed mismatch b/w the memory and the processor.