Try installing any linux distro that contains XFree86 version 4.0.3 or above. It should recognize a Radeon chip and your correct amount of video memory (it´s a generic driver for all radeon based video cards).
Try RedHat 7.1, or SuSE 7.1 - 7.2, or Mandrake 8.0 (the last one has been the easiest distro to install I have ever tried!)
Or you can download XFree86 and install it in your system, but I´m not sure how to do that.