With the Diamond Radeon HD 4850 it had worked out of the box with Catalyst 8.6 on Ubuntu 8.04 LTS.
CrossFire is not officially supported until next quarter and at that time we'll share the performance benchmarks from these two Radeon HD 4850 graphics cards. To configure CrossFire on Linux will be a new set of aticonfig options.
ATI PowerPlay and dynamic clock-gating are supported with the Catalyst 8.6 driver on Linux for the Radeon HD 4800 series. We had measured the AC power consumption of our test system with a lone Radeon HD 4850 512MB and then with a NVIDIA GeForce 9800GTX 512MB. While idling with the Radeon HD 4850, the system had consumed about 18 less Watts than the 9800GTX. When the system was under load, the power consumption delta was at approximately 56 Watts. While PowerPlay is supported on Linux with the Radeon HD 4850, if you run aticonfig --lsp to list the PowerPlay states you will experience an error saying that PowerPlay isn't supported. PowerPlay is working but as it's hardware-controlled with the RV770 GPU the states aren't adjustable to the end-user (compared to earlier Radeon products), but is dynamically controlled.
The Radeon HD 4850 supports Custom Filtering Anti-Aliasing (CFAA), Multi-Sampling AA, and other advanced AA features, however, they are currently not available on Linux. While there is the shared code-base now between the Windows and Linux drivers, on Windows these advanced AA options are only supported with the Microsoft DirectX API. If and when AMD brings these rendering features to OpenGL, we should see the support appear on Linux. For now, the anti-aliasing is limited to 8x and anisotropic filtering maxes out at 16x.