Today 7.2.0 version of xf86-video-ati was released (
http://lists.x.org/archives/xorg-announce/2013-August/002288.html). Version in Debian archive (6.14.4-8, last update in March 2013) is in a sense 5 versions old from last upstream release. Version 7.2.0 (and, of course, previous versions) adds a support for a lot of hardware. Please, update Debian version.