When you connect an Apple Cinema Display (30-inch DVI) to a PowerBook G4, you may find that the highest available resolution is 1280 x 800, even though the optimum resolution for the display is 2560 x 1600.
This is normal unless you are using a PowerBook G4 that comes equipped with a dual-link DVI port, such as:
These models were introduced in 2005 and come with the ATI Mobility Radeon 9700 video cards. Dual-link DVI ports were also available as Configure-to-order options in some other PowerBook G4 computers. Only these models of PowerBook G4 are able to support the optimum resolution for the Apple Cinema Display (30-inch DVI).