For processors that are built with the same generation of internal architecture, clock speeds are comparable and significant. For processors of very different architecture, they aren't.
Most programs are single-threaded, meaning that they can only run on one core at a time. For these programs, the speed at which the program runs depends on the clock speed of the processor. A program running on a processor at 3.0 GHz will run twice as fast one running on a processor at 1.5 GHz. Having dual cores won't help, because these programs can only run on one core at a time.
However, the situation changes if you are running programs that are designed to execute multiple threads. A program that has two threads internally, for example, can run on two processors at the same time. So if you have a dual-core processor, the program can use both cores at once. In this case, having two processors at 1.5 GHz (2 x 1.5 = 3 GHz total) will give you better performance with this program than one core at 2.0 GHz (1 x 2.0 = 2 GHz total).
Having two or more cores can also make the system more responsive. With two cores, and given that most programs can only use one core at a time, you have fewer problems with programs that hog a processor for themselves. If you have two cores, a program might tie up one core completely, but as long as it has only one thread, the other core is unused, and remains available for the rest of the system. This keeps the system responsive even when you're running a program that pegs a processor.
So it all depends on what kinds of programs you run. If they are multi-threaded, they'll perform better on multiple cores than on a single core, and their total performance will be roughly in line with the total horsepower of the CPU (n cores times x clock speed). So a dual core at 1.5 GHz will perform better than a single core at 2.0 GHz. But if most of your programs are single-threaded, you're better off with a single core at high clock speed than multiple cores at a low clock speed. A dual at 3.0 GHz will work better than a quad at 1.5 GHz, even though total horsepower is theoretically the same. The only advantage to multiple cores if you have mostly single-threaded applications is that you can run more than one application at full speed, or you can run one application at full speed and still have an extra core(s) for the rest of the system, which keeps your system from bogging down.
Be aware that this all applies for cores of similar architecture. With processors that use different architectures, the more advanced architecture is faster for a given clock speed. So an i7 processor is a lot faster than a Pentium 4 processor, even at the same or lower clock speeds.