At 1280 x 1024, the Radeon X800XL was pushing out just 30 FPS while the Radeon HD 4550 and Radeon HD 6450 on the open-source driver could no longer push a playable frame-rate. Fortunately, the Radeon HD 2000 series and newer are still supported by the proprietary Catalyst driver, which can make this game playable on the lower-end hardware. The top-end card tested was the Radeon HD 4850, which was doing above 60 FPS at 1280 x 1024. The Radeon HD 4650 and other higher-end GPUs were all above 30 FPS. The Radeon X1950PRO on the "R300g" (rather than "R600g") was running at an average of 44 FPS.
At 1920 x 1080, the X1950PRO, HD 4770, and HD 5750 had fallen to around 30 FPS while the Radeon HD 3850 and Radeon HD 4850 were both still above 40 FPS. Again, if you are really after performance and the best visuals, the proprietary NVIDIA and AMD drivers are the only suitable options at this point.
When the open-source Doom 3 source-code is out there, more benchmarks will be done when the drop has occurred and as the Mesa/Gallium3D drivers continue to march forward with greater OpenGL support, out-of-the-box S3TC texture compression, and other features. There will also be numbers compared to the binary blobs as well as image quality comparisons, all by the Phoronix Test Suite with OpenBenchmarking.org. Test requests are also always welcome via Twitter.
To sum up the situation for now, if wanting to run Doom 3 on open-source Linux graphics drivers, the best performance and stability is for now with the Radeon Gallium3D stack. There was never any lock-ups or other issues like was common for Intel and their classic Mesa driver on Sandy Bridge. Nouveau Gallium3D was also quite decent, but there seems to be a problem in the Fermi Gallium3D component delivering awful performance and overall the Nouveau support tends to be less optimized as AMD's open-source offering.