I had a similar issue with my Nvidia GT 440.
I prefer to fix this kind of thing from terminal. You can open one from the within Ubuntu by hitting Ctrl + Alt + T.
The first thing to do is get back to the nouveau driver. To do that, you'll need to clean out all the nvidia files you installed by running the following commands:
Don't worry if the command above reports that the file wasn't found. You're just making sure all the nvidia stuff is gone.
sudo nvidia-installer --uninstall
Now, restart your machine and the nouveau driver should be loaded automagically.
sudo apt-get remove nvidia-*
Once you're back at your desktop, you can either use the GUI mentioned in the previous post to install one of the proprietary nvidia drivers, or do so using Terminal.
With my GT 440, neither the nvidia-current nor the nvidia-current-updates drivers worked for me. The former displayed grub in 640 x 480 and caused problems with evolution-calendar-factory. With current-updates, the launcher wouldn't come out of auto-hide, but the nvidia-experimental-304 driver seems to work just fine. To install it from the command line, use the following commands. The first makes sure you're up-to-date, the second installs linux headers which are a dependency of the nvidia proprietary drivers, and the third installs the experimental 304 driver:
sudo apt-get update
sudo apt-get install linux-headers-generic
After that, restart to use the new driver.
sudo apt-get install nvidia-experimental-304