Argh, ok, yeah...
*ahem* It's real simple: Either use the nVidia installer, or the Debian packages, but not both!
Get rid of one before you install the other!
And I just found this out the hard way.
Finally with this thread I figured it out, thanks to netllama's mention of Debian's /etc/init.d/nvidia-glx script that I had noticed, which, sure enough, was the problem.
So, to fix
A. Remove the nvidia-glx and nvidia-kernel-* packages, and, if necessary, rerun the nVidia installer.
Or, B. Do it the Debian Way:
1. Uninstall the nVidia installer (if you want; I didn't and it worked).
2. apt-get install nvidia-kernel-source/experimental (use experimental to get 7676)
3. apt-get install linux-headers-`uname -r` (from a terminal; the `uname -r` will put in your current kernel; if you already have the headers for your kernel, skip this step)
4. apt-get install module-assistant (if you don't already have it)
5. sudo m-a auto-install nvidia
6. apt-get install nvidia-glx/experimental (again, experimental for 7676)
7. If you're running X with the nv driver at the moment, sudo dpkg-reconfigure xserver-xorg (if you have any custom changes to xorg.conf, like for a mouse, etc., you'll have to readd them); select the nvidia driver when it asks you which you want to use.
8. Restart X (Ctrl+Alt+Backspace)
I think that should do it. If just restarting X doesn't do it, reboot.
If you need more help, here's where you should go for a complete explanation: