Not sure if any of the supported chipsets come with DPort when used on PCIe cards,

But higher res would probraly still use some work to get the timing right.

Can't really give you a list of D-DVI cards, but I'm useing a Radeon 9650 that orginally came with a 2x2.7GHz G5 (mine is only 2x2.3GHz). This card won't work in a G4 without modifications (cutting some wires to disable AGPx8) not 100% sure if it would work after those mods.

I also have a G4/5 compatible 9700Pro which I'm not 100% if it does D-DVI, as It won't fit my modded G4 (the DVI-connector is slightly misaligned).

There are some other cards that should work, and with some tricks you can even get most PowerBook to output that res (tricks for use in MorphOS, works out of the box in OSX).