CentOS official repositories don't contain packages for NVIDIA closed source driver so one needs to add a third party repository that has the driver available, such as the ELRepo repository.
Adding the ELRepo consists of importing the public GPG key and installing the repository:
Having added the repository we can finally install the driver (yum pulls nvidia-x11-drv as a dependency) and reboot:
# rpm --import http://elrepo.org/RPM-GPG-KEY-elrepo.org # rpm -Uvh http://elrepo.org/elrepo-release-6-4.el6.elrepo.noarch.rpm
# yum install kmod-nvidia # reboot
By installing NVIDIA closed source drivers, the plymouth graphical boot will no longer be available. In other worlds, the pretty CentOS and SL logos on a black background will be replaced by a bottom horizontal progress bar.