check out this article from tweaktown.com:
http://www.tweaktown.com/articles/via_agp4x/
"If you have a motherboard based on the VIA Apollo 133A, KX133 or KT133 you may find sometimes (not in all cases) that you can't get your graphics card to run at AGP 4x within Windows 2000, especially when using NVIDIA reference drivers. With these drivers the registry setting for AGP 4x on VIA Apollo 133A, KX133 or KT133 chipsets isn't enabled.
Update: Due to an excess amount of TweakTown users asking how to do this same registry hack in Windows 9x we have updated this guide and included the registry hack for Windows 95, 98, 98se and ME."
Hope this helps.