jepoi: if your motherboard has no agp/pci clock, when you increase the FSB, the AGP and PCI clocks also increase. For example, if your FSB is 133 and you increase it to 150, your AGP clock (stock 133/2 = 66.7) will increase to 75. Your PCI clock (stock 133/4 = 33.3) will increase to 37.5.
AGP and PCI cards may not be able to handle the increased speed, and result in failures or glitches. If your motherboard HAS an AGP/PCI clock, the AGP bus stays at 66 and the PCI bus stays at 33 no matter what you speed your FSB is.
Hope this information helps you.