After our small conversation in the comments it appears that your problem is you haven't even been able to try installing the included proprietary driver yet, because you can't even run the installer (or, if you do run the installer without the graphics card present, you then cannot boot once you replace the graphics card).
Having virtually the same card as you, I found that for whatever reason, the nouveau drivers supplied with Ubuntu 12.04 just don't seem to work with this card - and don't allow it to boot into X properly (even for the graphical installer). Interestingly, the nouveau drivers included with Ubuntu 11.10 did work, so not sure what happened.
For the installer CD, there is an easy workaround - at the initial boot screen when the keyboard icon appears, press any key to get a menu, and find the menu option that allows you to boot with the nomodeset option. Without modesetting enabled, nouveau won't load (which means you'll get ugly, slow graphics, but this is only temporary!).
Since you have already managed to install Ubuntu by running the installer without the graphics card present, you don't need this step.
The next step is that you now need to install the proprietary driver on Ubuntu without being able to boot into X. You could try installing the package
nvidia-current. This can be done using the command line. There will be an option in your boot loader to boot to command line - if your boot loader doesn't come up then holding SHIFT while booting should do it.
I can't fully recall, but you may also need to install
nvidia-xconfig and run that, to set up your video driver for X after you've installed it.