Phoronix published a comprehensive AMD Radeon Linux graphics comparison, you can check it out over here.
"Last month I alluded to a 40-way graphics card comparison
being worked on at Phoronix. This comparison is to extensively compare
the performance of the open and closed-source drivers for each graphics
card and to comprehensively comment on other areas of the Linux graphics
driver support. Not only is the OpenGL performance being evaluated, but
the thermal performance, CPU utilization, and power consumption is being
looked at too. Being published today to mark the beginning of the
Oktoberfest 2011 articles are the ATI/AMD Radeon results. This includes
28 of the 40 graphics cards, with GPUs as old as the Radeon X800XL and
as new as the AMD Radeon HD 6950."