The same idea applies to the PCI Express bus. This bus uses a default clock rate of 100 MHz. If your motherboard doesn’t have a separated clock generator for this bus, you will have to increase the motherboard master clock generator, which will increase all clock rates used by all devices on the motherboard.

In Figure 17, you can see a motherboard without a separated clock generator for the PCI Express bus. The only possibility here is to increase the motherboard master clock generator, which will overclock everything connected to the motherboard.

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Figure 17: On this motherboard you can’t configure the PCI Express clock rate separately.

Some motherboards have a separate configuration for the PCI Express bus, but they don’t have a separated clock generator for the PCI Express x16 slot, which is the one used by the video card. Instead, they have a single clock generator for all PCI Express slots and connections, so when you increase the PCI Express clock rate, you will also overclock all devices that use this bus, including the ones connected directly to the motherboard, such as the SATA ports – and hard disk drives are very sensitive to any increase on the SATA clock. This is the case with the motherboard shown in Figure 18. It has a separated clock configuration for the PCI Express bus (“PCIE Clock” option), but this configuration will increase the clock rate of all PCI Express connections, not only the one used by the x16 slot.

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Figure 18: This motherboard has a separated clock generator for all PCI Express connections.

The best option is to have a motherboard with a separated clock configuration for the PCI Express x16 slot, like the one shown in Figure 19. On this motherboard, the clock configuration for the main PCI Express x16 slot is called “C51 PCI-Express Frequency.” The clock that will be used by the other PCI Express connections is called “MCP55 PCI-Express Frequency” and should be left at 100 MHz.

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Figure 19: Motherboard with separated clock configuration for the PCI Express x16 slot 

So, how do you determine the maximum clock rate your AGP or PCI Express x16 bus will support? Like everything related to overclocking, you do so by trial-and-error. Increase the clock rate a little, save the changes and exit setup, load Windows, run a 3D game like Quake 4 in its benchmarking mode, and see if the system remains stable. If it does, go back and increase the clock rate a little more, and repeat the entire process until you find the maximum clock rate your I/O bus will run without crashing the system.