I also have a 6900 card myself and it is an asus. On mine there is a switch on the graphics card which turns one display port of to switch the DVI from single link to dual link.
Both the DVI outputs are DVI-I but i think only one is truely DVI-I (dual link).
I have linked my specific graphics card and the picture that demonstrates where the switch is.http://uk.asus.com/Graphics_Cards/AMD_Series/EAH6950_DCII2DI4S2GD5/http://uk.asus.com/websites/global/products/mMAaEwb08yxZPzVe/sixmonitor02.jpg
The switch is circled in red.
I made an account just to answer this question xD
EDIT: DVI-D (dual link) will run from DVI-I (dual link) no problem