Is this error the one you get after modprobe'ing nvidia? Or before?
I have a feeling the issue that's causing the need to modprobe nvidia manually is that you booted to failsafe when you installed the drivers. In Mandrake, failsafe doesn't mount devfs (the device filesystem -- devfs is a completely in-memory filesystem; failsafe mode uses the on-disk device files instead), so the installer doesn't put the proper alias in /etc/modules.conf (the alias depends on whether your distro uses devfs or not; if it does, the alias should read alias /dev/nvidia* nvidia, and if not, it should read alias char-major-195 nvidia instead). So the kernel module doesn't load when you're back in real (non failsafe) mode.
Think of it this way -- you wouldn't install a video driver while booting Windows to safe mode, would you? I know I wouldn't... though I don't know exactly what all the differences are between Win normal and Win safe, I do know one very important one in Mandrake.
But if you're still getting that error after you modprobe nvidia, then you need to look in your kernel logs after an X attempt (use dmesg to see the kernel logs). It should have a reason in there that it failed.