All the information about the option you can use in your XF86Config file are in the README file from the NVIDIA driver.
nvAGP=3 means it uses the agpart module if available, the nvidia agp system in the other case. So this setting is fine. don't worry.
The fishy thing is you AGP disabled.
Sound in ET is in theory another pb...
Could you post us the log file of xfree and check out the content of the /proc/driver/nvidia stuff?
Well... from the readme file, with via chipset we may have to use a few tricks to enable AGP.