In most cases, you have to blacklist the nouveau video driver in order for the nvidia driver to load properly by adding
rdblacklist=nouveau to your kernel boot line in /boot/grub/grub.conf. Perhaps you didn't perform that step, which is outlined in
this guide.. You might also check whether your nvidia video card is supported by the current version of the nvidia driver, and if not, determine which available older version does. The guide I linked to will help you with that also.