Mhz / Ghz merely refers to the clock speed of the CPU. But there are many other factors that affect speed as well in a CPU. Generally the faster the clock, the faster the CPU, but this is not true always.
With the XP SERIES cpus, the name of the CPU was matched to indicate a speed comparison to Intel competition CPUs. an ATHLON XP 3200+ was equivelent to an Intel running at 3.2 Ghz. This is NOT TRUE for the Athlon X2 series CPUs. The name means nothing - just a model #.