I think the FSB is what the PCI-E and CPU bus use to derive their clock speeds...not sure on this...all I know is that you can raise your FSB to overclock your CPU...something I have done before
Frontside Bus... The FSB connects the Computer's Processor to the System Memory (RAM) and other components on the motherboard. These components include the System Chipset, AGP card, PCI & PCI-E Devices, and other Peripherals. Because the FSB serves as the main path from the Processor to the rest of the Motherboard, it is also called the "System Bus"
Be careful overclocking the FSB. By doing this you are overclocking everything not just the processor. This would include any PCI and PCI-e cards, chipset, memory, even the IDE and/or Sata controller.
The processor also runs on a multiple. So whatever the multiple is you will be increasing its speed by a divisor based on how much you overclock the FSB. For example a multiple of 13 on the processor with a FSB overclock of 10Mhz would increase your processor by 130Mhz. Also modern FSB works on dual data rate so you would be increasing the FSB X2 respectively. I honestly would not recommend overclocking the FSB unless you have a motherboard designed for it that you can change the PCI bus multiple with. Otherwise you can find yourself with a lot of dead video cards and even a dead motherboard.
You might get more bang for your buck just changing the CPU multiplier. That way you will get the added performance without sacrificing everything on the system bus.