As we all know, or should, Nvidia driver 5.16 disables AGP 4X in place of AGP 2X.

Entering this registry modification will re-enable AGP 4X when using 5.16. Give it a try..

[HKEY_LOCAL_MACHINESoftwareNVIDIA CorporationGlobalSystem]
"EnableVia4X"=dword:00000001