Nothing will work if the NVIDIA kernel module does not function properly. If you see anything in the X log file like "(EE) NVIDIA(0): Failed to initialize the NVIDIA kernel module!" then there is most likely a problem with the NVIDIA kernel module. First, you should verify that if you installed from rpm that the rpm was built specifically for the kernel you are using. You should also check that the module is loaded ('/sbin/lsmod'); if it is not loaded try loading it explicitly with 'insmod' or 'modprobe' (be sure to exit the X server before installing a new kernel module). If you receive errors about unresolved symbols, then the kernel module has most likely been built using header files for a different kernel revision than what you are running. You can explicitly control what kernel header files are used when building the NVIDIA kernel module with the --kernel-include-dir option (see `sh NVIDIA-Linux-x86-1.0-6629-pkg1.run --advanced-options` for details).
Please note that the convention for the location of kernel header files changed approximately at the time of the 2.4.0 kernel release, as did the location of kernel modules. If the kernel module fails to load properly, modprobe/insmod may be trying to load an older kernel module (assuming you have upgraded). cd'ing into the directory with the new kernel module and doing 'insmod ./nvidia.o' may help.
Another cause may be that the /dev/nvidia* device files may be missing.
Finally, the NVIDIA kernel module may print error messages indicating a problem -- to view these messages please check /var/log/messages, or wherever syslog is directed to place kernel messages. These messages are prepended with "NVRM".