It's been mentioned before, but I'll say it again.
Having 2 2GHz CPUs does not give you 4 GHz.
Having 2 processors means that both processors are running at the same speed but are sharing the tasks amongst one another. In other words, having two 2 GHz CPUs means that the CPUs share tasks amongst one another at 2 GHz. The benefits of having multiple CPUs is multitasking and multimedia apps that support SMP (symmetric multiprocessing).
The new dual-core CPUs are built on the same idea, except that both cores reside within the CPU die, meaning it can share tasks more quickly without the bottlenecks or latency of a system bus.