One of the most important topics beside the processor speed is the bus clock of a CPU. The clock speed at which the Front Side Bus (that's how Intel calls it) and the system is working has reached 133 MHz today. Some time ago, Tom already dealt with this topic and proved impressively that rising the bus speed can lead to better performance numbers than just using higher clocked CPUs (
Bus Speed History
In the beginning of computer history, there was no difference between the internal and external clock speed. A 386 DX 25 worked at 25 MHz processor clock and was able to communicate at the same 25 MHz with its page mode DRAM. In the beginning of the 90s it became obvious that processor speeds would rise faster than clock speeds of the main memory (best example was the 486 DX 50) and upcoming synchronous bus systems. Since the performance losses at lower clocked memory were tolerable, Intel introduced a new series of processors: The 486 DX2. The "2" means that the processor is clocked twice as high as the memory/bus. A 486 DX2 - 66 worked at 33 MHz memory/bus and 66 MHz processor clock.
Thus it was possible to use less expensive memory chips. Thanks to the increasing use of L2 caches, performance drops caused by the lower memory clock were small. The main reason for this "clock splitting" was the introduction of the VESA Local Bus and the first PCI systems, offering much faster bus performance for graphics and I/O components. While PCI was specified for only 33 MHz, VLB was specified for 40 MHz. To obey it, processors were clocked at double (486 DX2-50, DX2-66) or triple bus speed (486 DX4-100).
With the introduction of the first Pentium computers, the internal and external clock speeds had been equalized for the last time: Pentium 60, Pentium 66. Following processors (Pentium 75, 90, 100) worked with a multiplier of x1.5 and different bus speeds (50, 60, 66 MHz).
Today's computer architectures have been optimized, so that some computers do already work with multipliers of up to x8 (Pentium III 800 at 100 MHz bus or Celeron 533 at 66 MHz bus clock). However, performance increases cannot hold up to the clock speed rising as long as the bus speed is not pushed up as well.