You may find that the easiest way to get it working is to do the following:
As root edit the file /etc/X11/xorg.conf (make sure you keep a copy first)
You will find a "Device" section which may well have a standard vesa driver
listed.
i.e. the lines
Section "Device"
Identifier "Videocard0"
Driver "vesa"
Vendorname "......
Should be changed so that the Driver line is Driver "radeon"
You can change the Vendorname to "Videocard vendor"
and also the Baord name to what you want.
If you save the file with the amendments and then re-start X (either by
doing telinit 3 and waiting for the system to stop X, then telenit 5 (all
as root) in a virtual console or by ssh into the machine from elsewere.
Once X has re-started you should be able to log in again with the new
driver being used.
If this is a problem just re-boot and the system will use the new driver
once you have booted back up. Now you should have the new driver in use.
For more information type "man radeon" in a terminal window. This will tell
you if your card is supported by the driver and list lots of other info.
I recently did this for a different radeon card and it worked fine.
Hope this helps
Mike
