NordicHardware compiled a guide that shows you how to overclock Radeon HD 2900 XT when running two of them in CrossFire mode.
How to overclock Radeon HD 2900 XT CrossFire
Usually you would need BIOSes and a flash tool to do this, but we've got a way to work around this.
How to overclock Radeon HD 2900 XT CrossFire
