Dual core and HT processors will perform better than single cores and accept parallel data streams. Pentium 4's and Pentium D's are built around NetBurst architecture, so a higher clock speed for those series is beneficial. Core 2 and Penryn are based on the Core architecture, meaning that a higher GHz rating does NOT guarantee superior performance.
The speed of a processor determines how fast it can interpret instructions it receives. A high FSB (front side bus) speed can improve the clock speed of the CPU.
EXAMPLE: Pentium D @ 3.2 GHz w/ 800 FSB is not as efficient as a Core 2 Duo @ 2.8 GHz with 1333 FSB.
Remember that the processor also needs adequate RAM to function to its potential.