Phoronix: An Open-Source Radeon HD 4670? Sort Of.

When the Radeon HD 4850 and Radeon HD 4870 were introduced earlier this year, it was wonderful. These latest high-end graphics cards from ATI had same-day Linux support through their Catalyst driver and the open-source ATI drivers had "just worked" with the RV770 series. The mode-setting support with the xf86-video-ati and xf86-video-radeonhd drivers just required adding in the PCI IDs for these new PCI Express graphics cards and then the rest of the magic was provided by AtomBIOS. However, with the introduction of the Radeon HD 4600 series, not everything is working instantly with the open-source drivers.

http://www.phoronix.com/vr.php?view=12850