Set the resolution in system preferences FIRST.

Also ensure that 'set screen to resolution xx at startup' is not enabled in the catalyst control window for the relevant screen

Be aware that depending upon your graphics card, you may not be able to achieve 3072x768 in catalyst, despte being able to see it on the Mac desktop. Its due to limitations with the OpenGL texture size on the graphics card (i think...)

I have tried with that resolution on a 2.0 and 2.7G Dual G5 both with 9800Pro and X800 graphics cards. Mac Desktop will display the resolution - Catalyst will not. I had to settle for 2400x600 - which was fine, as I was scan-converting to SDI.

Not all the resolutions listed for Dual and Tripple Head are achievable in catalyst (depending upon graphics card) - refresh rate can also be a limiting factor. I may post some test results here soon...