To set a new clock speed within the supported clock range, run aticonfig --od-setclocks=CORE,MEM where CORE is the integer clock frequency being requested for the GPU and MEM is the integer memory clock. If either MEM or CORE is set to 0, it will use the existing clock value. This will immediately set the graphics card to run at these new frequencies.
Once the system has been running stable at these frequencies, aticonfig --od-commitclocks can be run. Once the clock frequencies have been committed, every time X starts it will attempt to use the committed core and memory frequencies. In comparison, right now nvidia-settings needs to be run each time the user logs in (or by using an automated script) for any overclocked values to be set automatically. If the system isn't stable, aticonfig --od-restoredefaultclocks can be run to restore the frequencies to the default clocks defined in the video BIOS.
From our testing of ATI's OverDrive on Linux with the different Radeon HD 4800 graphics cards, we have been pleased with the results. By just increasing the core and memory frequencies by a few megahertz can lead to a gain of a couple percent when it comes to OpenGL frame-rate. Overclocking using aticonfig isn't as easy as moving a slider on a GUI window, but soon enough we will hopefully have OverDrive integration in the AMD Catalyst Control Center. Unlike NVIDIA's CoolBits, there is also no option to find the ideal clock frequencies for the graphics card. It is also nice finally having formal support for thermal monitoring and reading the GPU usage.
We'll also be shipping a Phoronix Test Suite module that automatically runs various OpenGL programs and will automatically overclock the graphics core and memory frequencies by a few megahertz each time until it finds the point of losing stability. The Phoronix Test Suite though does already support reading the OverDrive frequencies during testing as well as the GPU temperature/load. In the mean time though, the Catalyst Linux driver does ship with atiode, which is an ATI OverDrive stress application. Running atiode -P 60 would attempt to stress the ATI graphics processor for 60 seconds to check stability. However, this utility isn't the best so we would recommend using the etqw-demo, lightsmark, or unigine-sanctuary test profiles with the Phoronix Test Suite.
It's wonderful for enthusiasts to finally have ATI OverDrive on Linux. However, with this initial implementation you will be bound to using the command-line aticonfig until these options are integrated with the Catalyst Control Center, which hopefully isn't too far out in the future. Another shortcoming is that this OverDrive support is limited to the Radeon HD 3000 series and later, but earlier R600 and R500 GPUs will not be supported. Stop by the Phoronix Forums to share your overclocking results or if you run into any problems.