What's the output of 'uname -r'? NVchooser.sh may have given you the wrong RPM to install. If so, please contact me via PM so I can get a bug report from you.
To uninstall an RPM, you have to pass the exact name listed in the RPM database. This doesn't always match the actual name of the RPM. (annoying, isn't it?) To figure out what the name is, you can run 'rpm -qa | grep "NVIDIA_kernel"'. If the RPM you installed is the only one returned, then you can either cut and paste the result to 'rpm -e *RESULT*', or do 'rpm -qa | grep "NVIDIA_kernel" | xargs rpm -e'.
Once the 2.4.18-18 RPM is uninstalled, you should be able to install the one you rebuild yourself.