I have an ATI Radeon x1300 graphics card which worked wonderfully under windows, I was able to play Halo with all the neat effects turned on, etc.
Now that I've switched over to Ubuntu, and am using the non-proprietary driver for it, anything 3d has slowed to an unberable crawl, usually less than 1 fps if it fills up most of the screen. If it's small, for example, glxgears in its default size, and most of the screensaver previews, it runs great. glxgears at default size runs over 100fps. Resize to fullscreen, and the fps drops to 10, and screensavers seem to drop to 1 fps.
I'm really hopeful that there's something I can do to fix this without resorting to the proprietary driver (which I wasn't able to get working anyways, not to mention if I wanted to go proprietary, I'd just return to Windows). Don't make me pick between Linux and 3d, because I'll pick the latter.