If you have the AGP speed limited in your motherboard BIOS then you will have to adjust it from there. If the BIOS allows AGP 8X then you should make sure your motherboard chipset drivers are installed (instructions here) and then use one of the various NVIDIA utilities to set the AGP speed. This page is about underclocking your AGP but it shows how to use the various utilities to control your AGP speed. By the way, if your AGP speed is currently set to AGP 4X then you're not going to gain much of anything going to AGP 8X.
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