Using Ubuntu 9.04 on AMD64, and tried to install nVidia's beta 185 driver for my card, the GeForce 8400 GS (I do know that my card is supported by this version; this isn't a hardware issue) but after using the proprietary drivers dialog to install the driver, I rebooted to a terminal- since I didn't get a little message telling me that I needed to make a new xorg.conf, I did some looking around and it appears that xorg had been uninstalled. I tried installing xorg, and it conflicted with the package nvidia-glx-185. I tried to get rid of that but it wouldn't let me. I booted a livecd and chroot'd in so I could paste the exact error message. It is as follows:
It won't let me install xorg with nvidia-glx-185 installed, but it won't let me uninstall that. So, what next?

EDIT:
I was able to uninstall nvidia-glx-185 by extracting a file called libGLcore.so from the .deb for nvidia-glx-185 to the path it expected and trying to uninstall it. Then I installed ubuntu-desktop everything worked once more.

for the nvidia.run driver/installer it DOSE NOT replace your xorg1.5 It only replaces 2 opengl files( .so) and 2 headers but reinstalling your mesa and mesa-dev will take care of the .01% of programs that will not work with the nvidia opengl. ( my custom built Celestia 1.6 ( svn) is one of them )
To uninstall the .run driver run