As we had reported last week with these two Radeon HD 3800 series graphics cards, they aren't yet officially supported by the open-source RadeonHD driver but they do function with the Catalyst 7.11 driver for Linux (and are just identified as being an "RV670"). However, on the Phoronix Forums, we have seen reports of some users running into problems. The RV670 is based upon the R600, but the good news is that the Linux community has finally been greeted by same-month support in the proprietary Linux driver. This turn around time is in sharp contrast to the past where we have had to wait some time for the GPUs just to be supported (NVIDIA has the same problem with some product releases and its Linux support).
The good news is that the Catalyst 7.12 driver, which will be released this month, improves the Radeon HD 3850 and Radeon HD 3870 support and we will share more on that once the driver launches for the public. On the open-source side, we expect the RadeonHD driver will properly support the Radeon HD 3850 and Radeon HD 3870 in the near future.