To make sure that your computer can run Unity 3D, type:
If Unity 3D can be supported, you should have "Yes" responses to each of the items tested. It seems like you are probably going to get some "No" responses; please post the output here so we can get a better idea of the problems.
This is the output from mine:
Are you running the default drivers, or the Nvidia proprietary drivers? Open System Settings (under the "Gear Wheel" in the upper-right corner of the screen), and open Additional Drivers. It might be that there is a proprietary driver available if you enable it there.