The fglrx driver is not known for its speedy 2D or 3D performance, but there are ways to improve it.
If you suffer from slow 2D speed it might help to set
Option "XaaNoOffscreenPixmaps" "true"
in the fglrx device section of /etc/X11/xorg.conf.
This might also help the pixel errors in the lower right corner that occur on some machines. It disables 2D acceleration almost completly and let the CPU do all the rendering. Hopefully the driver will provide better XRender acceleration soon.
Is required to run compiz.
If you plan to run Wine, you will need to tell fglrx to not corrupt the fs register. To do this, add the line:
Option "UseFastTLS" "2"
to the "Device" section in /etc/X11/xorg.conf. This supposedly causes about a 10% hit in opengl benchmark scores, but at least things don't crash. See also http://wiki.winehq.org/fglrx and http://www2.ati.com/drivers/firegl/readme0325.txt