Just a quick note for those googlers out there again. My sister has my old and trusty Radeon 7000 (RV100) GPU in her machine and after distro upgrade, we had trouble getting it working with respectable performance; DRI etc. was all fine after the usual installation round of firmware-linux, but heavy 2D blipping work (e.g. moving windows, OpenTTD, darkening/lightening whole screen in various applications) was taking awful amounts of CPU within Xorg and things were really choppy.
It seems that there are some bugs lurking within the KMS; radeon.modeset=0 on the kernel commandline has a good shot at curing things like this!