AMD Radeon HD 5750/5770
With the Lightsmark 2008 test profile, the ATI Radeon HD 5770 graphics card remained the fastest on Linux among the ATI GPUs that were tested. The Radeon HD 5750 was ahead of the Radeon HD 4850 by 14%.
Taking a quick look at the 2D performance, the GtkDrawingArea Pixbufs numbers were more or less the same between all of the ATI GPUs.
The best X Render performance measured through QGears2 with image scaling was found actually on the Radeon HD 4670 followed by the Radeon HD 4870.