Alot of times the modules.conf file must be edited to load the NVdriver. The Install should modify the modules.conf file for you however many times it does not. A second thought is that most your xconfig files are in this format XF86Config-4 for xfree86 4.0 and newer and the XF86Config file is for the old version xfree86 3.0 versions and NVdrivers dont work with the older version.
? are you editing right XF68Config-4 file
? is your modules.conf file loading NVdriver
? what distro are you running and what are you running it on