nouveau, the project for creating a free 3D linux driver for nvidia cards, recently got first support for real 3D applications with gallium on some NV4X cards (see Nouveau Companion 36). Today I got it working on a friends machine.

Here you can see an openarena benchmark (also uploaded on youtube). It got 55 fps, which is far away from the nvidia binary driver yet (178 fps), but at least more than my r200 setup (32 fps).

For the brave ones, here's a quick and dirty howto for Gentoo:
a) Get the nouveau overlay with svn co https://svn.hboeck.de/nouveau-overlay and add it to PORTDIR_OVERLAY in make.conf.
b) The nouveau-overlay won't install the nouveau/gallium-branch of mesa. Get my overlay with svn co https://svn.hboeck.de/overlay and also add that to your PORTDIR_OVERLAY (I'll try to contact the nouveau-overlay developer if we can merge this).
b) Add media-libs/mesa, x11-base/x11-drm, x11-libs/libdrm and x11-drivers/xf86-video-nouveau to /etc/portage/package.keywords and merge them.
c) If you've been running the nvidia binary driver, eselect opengl set xorg-x11, change the graphics driver in xorg.conf to nouveau, rmmod nvidia (if you've been running the binary driver), modprobe nouveau and start X.
d) Have fun!

Note: The nouveau developers consider gallium completely unsupported at the moment and don't want to get end-user bugs. If it runs, fine, if not, don't nag them with it.