DVI-D Dual link

[Kodoss]

Your DVI-I connectors should be also dual link, or you wouldn'd be able for 2560x1600 with one DVI-I Cable.

Just buy a DVI-D Dual link cable put it on the monitor and into your DVI-I slot of your graphics card.
Don't try to put the DVI-I cable into DVI-D slot of the monitor. It will not work, because the ground pin by DVI-I is bigger than DVI-D Slot.

In Short:
DVI-D cable(pins) + DVI-I graphic card(slot) = OK
DVI-I cable(pins) + DVI-D monitor(slot) = false

PS: I also have a DVI-D cable on my HD6990(DVI-I) because the monitor has only DVI-D. Res: 1920x1200.