What is the most common CPU speed?

This can be measured in a lot of ways. Typical for some categories listed below:

Typical home-end clock frequency is 3.0-3.6 GHz without any kind of overclocking.

Typical MiPS is averaging nearly 100,000 (This is getting phenominal!)

Processors are approaching the TFLOP level on the home-end as well.

IPC is 30~ for AMD and 50~ for Intel.

The last 3 are far more important in determining a processor's speed than the clock frequency.