I had the same problem (Geforce 2 GTS and nvidia-glx-legacy) and it was the modification of xorg.conf, not the newest mesa-utils that seem to fix it.
-- Xlib: extension "GLX" missing on display ":0.0" https://launchpad.net/bugs/63461 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
