hmm.. did you install the nvidia driver properly (you'll need to restart X11 (or the computer), btw)? What do you get in 'glxgears' now?

As fr doom:
Download this file and save it (don't install it yet).
Then run, as root:
mkdir /usr/local/games
mkdir /usr/local/games/doom3
mkdir /usr/local/games/doom3/base

Then mount the win32 install cd's, one by one, and copy the pak000.pk4, pak001.pk4, pak002.pk4, pak003.pk4, pak004.pk4 from the base/ directory inside each cd, into /use/local/games/doom3/base as root.
Then run the installer as root. 'cd' to the directory where you saved the file, and run './doom3-linux-1.3.1302.x86.run'. Follow the wizard.
Then run 'doom3' to play..........


Tuxracer runs on GNU/Linux, mac OS X, and Windows BTW. It should be in the repositories, so search for it in Synatic and install the relevant packages.