"Now, however, we finally have real (and official) ATI
overclocking support on Linux. ATI OverDrive 5 is now supported
with fglrx 8.52 / Catalyst 8.8 for the Radeon HD 3000 and Radeon HD
4000 series. This first-cut OverDrive support is only available
through the CLI-based aticonfig utility, but in a future driver
release, they will add an overclocking area to the AMD Catalyst
Control Center Linux Edition."