Unfortunately Ubuntu doesn't support the old drivers, they recommend to switch to the free Nouveau drivers for your card, but the shipped Nouveau drivers support only 2D

You could try the experimental 3D-enabled Nouveau drivers from the
xorg-edgers ppa, but this is _experimental_ software.
Also the TNT2 is very old, I expect it to do no more than 10-20 fps anyway. Have you considered upgrading to a new(er) graphics card (you don't need to buy a 5970, every low-end GPU is much much faster than the ancient TNT2).