As we mentioned, another option you have is to overclock the bus where the video card is connected to AGP or PCI Express, depending on your motherboard.

NOTE: Depending on your video card, you may not notice any performance gain by overclocking the I/O bus. This occurs because you may already have enough free bandwidth on the I/O bus, so it is not creating a bottleneck for your 3D performance. It is very important that you run games in their benchmarking modes before and after playing with the I/O bus in order to check whether you had any real performance gain by overclocking it. If you don’t get any performance improvement by tweaking the I/O bus, it makes no sense to keep it overclocked. In this case, leave it back on its default configuration.

We, however, encourage you to at least try playing with the I/O bus if you want to extract the maximum performance from your video card and then, at the end, determine if it was worthwhile.

Here, the options you will have will depend on your motherboard model. We will explore all possible options; however, your motherboard may not have all of them available.

The main problem here is that only overclocking-oriented motherboards will have a separated clock generator for the AGP or PCI Express x16 bus. On simpler motherboards, a single clock generator is used by all the devices found on the motherboard. If you want to increase the AGP or PCI Express x16 clock rate, you will have to increase the master clock generator rate, which will automatically increase the clock rate used by all other devices.

The problem with this setup is that everything will work overclocked as well, not only your video card. Thus, you may face a situation where you won’t be able to pass a certain clock level, not because your video card can’t go over it, but because some other devices on your system that are also overclocked have reached their clock limit.

The following are some examples of how to overclock your AGP or PCI Express x16 bus.

First you will need to enter your motherboard setup utility, which is achieved by pressing the Del key right after you turn on your PC. Inside the setup, you will need to find where the overclocking options are located. The exact location varies according to the motherboard model. Please note that on some motherboards you need to change some configurations from “auto” to “manual” in order to see the overclocking options.

In Figures 15 and 16, there are two motherboards based on the AGP bus. The AGP bus runs at a default clock rate of 66 MHz.

The first motherboard (Figure 15) uses a single clock generator, and to overclock the AGP bus, you need to increase the CPU external bus. By increasing the CPU external bus, you will overclock not only the AGP bus but also the CPU, the PCI bus and all other devices found on the motherboard. On this motherboard, you need to change the “CPU Host Clock Control” option to “Enabled” in order to have access to the external clock rate configuration (“CPU Host Frequency”). Note how you don’t have access to the option “PCI/AGP Frequency.” This option only displays the new PCI/AGP clock rates based on the new external bus configuration you entered.

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Figure 15: On this motherboard you can’t configure the AGP bus clock rate separately.

On the second motherboard (Figure 16), there is a separated clock generator for the AGP bus. We know that because there is an option called “Adjust AGP Frequency,” which defaults to 66 MHz.

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Figure 16: On this motherboard you can configure the AGP bus clock rate separately.