I can't say whether this will work on any other VIA mobos but it worked for me. My Viper V770 Ultra used to run at 2X. Now it runs as advertised at 4X AGP. SiSoft Sandra and Wcpuid confirm this. No stability problems at all!
[HKEY_LOCAL_MACHINE\Software\NVIDIA Corporation\Global\System]
"EnableVia4x"=dword:00000001
[HKEY_LOCAL_MACHINE\Software\NVIDIA Corporation\Global\System]
"EnableVia4x"=dword:00000001