Why would you link to an article about r600g when discussing HD5xxx cards? HD5xxx cards are r800. And they will likely have some 3D acceleration via Mesa in the next month or so (though there are no promises, of course).
The r600 classic mesa driver supports 3D acceleration on HD2xxx/HD3xxx/HD4xxx cards already (OpenGL 2.0), certainly well enough to play ut2004 and even doom3. r600g (which that article is about) should hopefully give a performance boost and include OpenGL 2.1 support.