Hi

>> Did you emerge nvidia-kernel yet?
>
> I just tried emerging the kernel and it worked, but I get the same error
> when I try to emerge nvidia-glx

I don't know why, but currently different versions of nvidia-kernel
and nvidia-glx are unmasked.

Try to emerge nvidia-kernel manually:

emerge /usr/portage/media-video/nvidia-kernel/nvidia-kernel-1.0.4363-r3.ebuild
emerge /usr/portage/media-video/nvidia-glx/nvidia-glx-1.0.4363.ebuild

-thomas