Download the latest nVidia drivers from here: http://www.nvidia.com/object/linux_display_ia32_1.0-9755.html
Reboot your box. I assume you are using Grub for the bootloader. Instead of launching Ubuntu normally, select the option that will cause you to boot to a command prompt (maintainence option I believe).
cd to the location of where you downloaded the driver. It you downloaded it from firefox it probably went into your Desktop folder.
Change to that folder like this:
run the nvidia installer like this
sudo sh NVIDIA-Linux-x86-1.0-9755-pkg1.run
This will bring up a bunch of dialog boxes, and you will want to reboot. If all goes well you should see a nice green NVIDIA logo appear after you reboot.