You can only compare CPUs based on clock speed if they are in the same family, like a Phenom II X4 945 @ 3.0 GHz will be slower than a 980 @ 3.7 GHz, but the Phenom II 980 @ 3.7 GHz will get beat by an Intel i5 2500 that is only running at 3.3 GHz.
Architecture contributes far more to how well a CPU performs than does its clock speed, we have hit the ceiling for clock speeds, they haven't increased much since in the Pentium 4 era, but the amount of instructions done per clock cycle increases with every generation of CPU. Tom's has a CPU hierarchy chart to help with comparisons.
Also, a dual core at 2 GHz has 2 cores that each run at 2 GHz, this is NOT equivalent to a single core that runs at 4 GHz. Each core runs a single thread(assuming hyperthreading is disabled), so if you are only running a program with a single thread a dual core isn't going to be able to do it faster than a single core which is why you don't just get to multiply cores by clock speed.