AMD 751 motherboards support up to AGP 2X and GeForce 2 MXs support up to AGP 4X so you can run at a max of AGP 2X. If you don't have motherboard chipset drivers installed then you're probably running at AGP 1X.
This page explains in detail how to install chipset drivers. Some motherboards with your AMD chipset use an AMD northbridge and a VIA southbridge (the page listed above explains what that means). If that's the case then just install the AGP driver rather than the AMD driver pack.