Legion X,
I don't have the VIA P4B400, however from reading the forums at Rage 3D, it would seem that NO ONE is getting the two to work together. Whose to blame VIA or ATI? Seems like both at this point. Although I'd expect drivers to fix this incompatibility will be release soon by both companies. Especially before the join ATI-NVIDIA-VIA AGP 8X validation event they are having.
-Mootorks