On Athlon motherboards with the VIA KX133 or 694X chip set, such as the ASUS K7V motherboard, NVIDIA drivers default to AGP 2x mode to work around insufficient drive strength on one of the signals. You can force AGP 4x by setting NVreg_EnableVia4x to 1. Note that this may cause the system to become unstable.
Many Via-based motherboards allow adjusting the AGP drive strength in the system BIOS. The setting of this option largely affects system stability, the range between 0xEA and 0xEE seems to work best for NVIDIA hardware. Setting either nibble to 0xF generally results in severe stability problems.
If you decide to experiment with this, you need to be aware of the fact that you are doing so at your own risk and that you may render your system unbootable with improper settings until you reset the setting to a working value (w/ a PCI graphics card or by resetting the BIOS to its default values).