With this initial support, however, you can expect a few caveats. Initially, this CrossFire implementation only supports the Radeon HD 4800 series (R700/RV770) but in the coming months the support will likely be added for the R600 series. In addition, only two GPUs are supported for CrossFire -- either a single graphics card with two GPUs like the HD 4870 X2 or using two Radeon HD 4850 graphics cards. Later on they may possibly add the four-GPU CrossFireX support. It's also important to keep in mind that invasive changes were required when adding this multi-GPU support, so don't be surprised if there are a few other issues. We'll talk more about the CrossFire support when the supported driver has been released.
There is no CrossFire support in any of the open-source ATI drivers for X.Org (xf86-video-ati and xf86-video-radeonhd) and don't expect it in the near future. Before any of the open-source developers are likely to work on the shared-rendering across multiple-GPUs support, first the kernel memory management (now mainly GEM) woes need to settle down, DRI2 returning to a pleasant state, and the Mesa 3D ATI component migrating to the Gallium3D framework for first delivering a reasonable level of performance. We also don't have any level of open-source 2D/3D R600/700 support quite yet. If you add in the current lack of (NDA-free) CrossFireX documentation and all of the other unmentioned prerequisites and items of higher priority, we would be surprised if there's an open-source CrossFire-supportive driver before 2010.
For those just interested in using one of the Radeon HD 4870 X2's GPUs with an open-source driver, you can do that today. The PCI ID for the Radeon HD 4870 X2 is 0x9441. This ID was committed to the xf86-video-ati master branch last night so if you check out the latest source-code and build the driver you can have this card up and running. The xf86-video-radeonhd driver had RV770 hard-coded support added while they are turning to the AtomBIOS abstraction layer. If adding the HD 4870 X2 PCI ID to the RadeonHD master driver, it will work with this Novell-produced driver. We expect the 0x9441 PCI ID though will be added to the RadeonHD driver very soon. However, due to the current lack of open-source R600/700 2D/3D/Video support, both drivers right now only provide mode-setting support with no hardware-based acceleration.
We will be sharing our Radeon HD 4870 X2 Linux results later this month as well as the Radeon HD 4850 and Radeon HD 4870 CrossFire test results from the Phoronix Test Suite.