/usr/lib. If you do not know how to paste file in restricted area, you can do it as root: open console (terminal), and enter command:
sudo cp /dir_where_the_file_was_downloaded/libGL.so.1.2 /usr/lib
It works immediately.
You may want to copy the file to safe name /usr/lib/libGL.so.1.2.good, and after the next ATI drivers update use your own copy.