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.