Radeon Gallium3D Has Made Much Progress In Two Years
Doom 3 unfortunately hasn't seen much change at all for this particular hardware on the R600 driver.
Lightsmark is certainly a different story with the performance having more than doubled in two years when comparing R600c on Mesa 7.9.2 to the soon-to-be-released Mesa 8.1 with Gallium3D. Even going from the current stable Mesa 8.0.4 release to Mesa 8.1-devel yields significant improvements from the Radeon HD 4870 graphics card.
The Nexuiz performance has nearly tripled in the past two years on the open-source Radeon Linux graphics driver.