I get the following information from http://www.leadtek.com/faq.htm and I hope it will help you. But the *.reg files are applied for Nvidia display card. I not sure whether it can be used on ATI Display card.

For users with VIA 4X AGP support chipset motherboards, if you can only run at AGP 2X mode, please check the followings
1.) Check your motherboard BIOS, make sure you have enable the AGP 4X support.
2.) Install the latest Drivers for your Leadtek Graphics card
3.) If the system is still running at AGP 2X mode. the following registry can force the GeForce driver to turn on 4X mode support. The 4X off registry will turn off the AGP support back to 2X mode. (after download the files, double click to install, restart to take effect)
For Win 9X/ME:
4XAGP on registry
ftp://ftp.leadtek.com/usa/via/w9x-viaagp4xon.reg
4XAGP off registry
ftp://ftp.leadtek.com/usa/via/win9x-viaagp4xoff.reg
For Win 2K:
4XAGP on registry
ftp://ftp.leadtek.com/usa/via/w2k-viaagp4xon.reg
4XAGP off registry
ftp://ftp.leadtek.com/usa/via/w2k-viaagp4xoff.reg
4.) Most VIA chipset motherboard BIOS included a function call "AGP driving value". Find this function and try setting this value to DA or EA or E7. This function is related to AGP timing. Tuning the value will help improve the stability.