They go on and on about this over at Rage3D. Last time I checked, it seemed that they had decided it was all about AGP chipset drivers.
Your ASUS has a non-Intel based chipset, I believe. What does the installation manual say about accounting for that? I can't figure it out. It's towards the front of the book. Maybe means nothing.
If it's showing an incorrect AGP value using the ATI drivers you are also going to get an incorrect AGP value using the Omega drivers.
Mine have registered perfectly every time in both 98se and XP. Probably 'cause I've got an Intel-based AGP chipset.