Common Options : 1/1, 2/3, 1/2
The AGP bus clock speed is referenced from the CPU bus clock speed. However, the AGP bus was only designed to run at 66MHz while the CPU bus runs anywhere from 66MHz to 133MHz (266MHz when clock doubled). Therefore, a suitable AGP bus to CPU bus clock speed ratio must be selected to ensure that the AGP bus won't run way beyond 66MHz.
When the ratio is set to 1/1, the AGP bus will run at the same speed as the CPU bus. This is used for processors that use the 66MHz bus speed, like the older Celerons.
The 2/3 ratio is used when you use a processor running with a bus speed of 100MHz. The ratio will cut the AGP bus speed down to 66MHz.
The 1/2 ratio was recently introduced with motherboards that provide 133MHz bus speed support. Such motherboards need the 1/2 AGP bus to CPU bus clock speed ratio to make the AGP bus run at the standard 66MHz. Otherwise, it would have to run at the overclocked speed of 89MHz, which is more than what most AGP cards can withstand.
Generally, you should set this function according to the CPU bus speed you are using. This means the ratios of 1/1 for 66MHz bus speed CPUs, 2/3 for 100MHz bus speed CPUs and 1/2 for 133MHz CPUs.
If you are overclocking the CPU bus, theoretically you are supposed to reduce the ratio to ensure that the AGP bus speed remains within specifications. However, most AGP cards can run with the AGP bus overclocked to 75MHz. Some even run well at 83MHz! However, anything above 83MHz would be a little iffy.
Thus, in most cases, when you overclock the CPU bus, you can still stick with the original AGP bus to CPU bus clock speed ratio. This means that the AGP bus will be overclocked as well. But as long as the AGP card can stand the higher clock speed, it shouldn't be a problem. In fact, you can expect better AGP bus performance when the AGP bus is overclocked.
Be warned though - overclocking the AGP bus can potentially damage your AGP card. So, be circumspect when you overclock the AGP bus. 75MHz is normally the safe limit for most AGP cards.