Dont let your ASUS 6600 work with full FrontSideBus 100 Mhz or 133 mhz. It wont work.
You habe to use the AGP port with specific 66 Mhz.
I had exactely the same problem. My AGP port worked with full front side bus. A replaced a necessaru jumper on the Mboard and it worked fine with each driver 3.75 - 3.79 - 3.68 - 5.13 in win98 or win2000