Could you check if you're using the i686 kernel.
The installer wrongly installed an i586 version on many systems. Since it's the same version number, yum can't update it.
You can fix this by installing the i686 one like this:
rpm -Uvh --replacefiles --replacepkgs kernel-2.6.18-1.2798.fc6.i686.rpm
As for your yum command, notice that the 8776 driver isn't compatible with compiz/aixgl. If you want to enable the desktop-effects, install the nvidia-kmod package from testing:
yum install kmod-nvidia --enablerepo=livna-testing