you might have to reinstall , depending on just how mangaled things are but
here is the SL6 guide
just use the centos reops
see the wiki
and install the elrepo repo
if you are trying to use the .run
you MUST !!!!!
100% MUST !!!!!!
rebuild the boot image after you uninstall the nouveau driver and BEFORE!!!!!!!!!!!
you try to install the nvidia.run
( and ALL of that must be done in a text only boot )
see the script i posted here
the .run driver will NOT install if the nouveau is STILL in the boot image
Also you have to have installed already
gcc and the kernel-devel rpms
yum groupinstall "Development Tools"
yum install kernel-devel
using the kmod-nvidia is a LOT easier