Progress!!

Starting from the login screen, I press control-alt F2 to bring up a command
line.  Then I run startx.  I don't get a window manager, but I can find my way
to xterm and then work from the command line after X starts.  glxgears looks
normal - smooth and as quickly as I believe the 9550 can go. :)  Opening
firefox, CNN and youtube videos play smoothly.

>From your comments, I believe I have sidestepped lightdm. I opened synaptic and
reinstalled lightdm and associated programs.  I then minimized synaptic and now
I know what the display manager does.  That's OK, I'll be rebooting anyway.

Next steps?  I believe there is /usr/lib and an /etc folders.  Can I clean them
out and do another re-install?  Will the config files get regenerated?

Thanks