This solution was partly right, only one of the DVI ports does support dual link, but on this card there is a little switch you have to switch over to turn it into a dual link dvi port. Once I did that it worked.
I have exactly the same problem, same card, same resolution, very similar motherboard! I have tried everything I could think of, but I am desperately blocked at 1600x1200 on a monitor that supports 2560x1600 (and works perfectly fine on my older PC with a 5850).
Could you please give me more information about that secret switch on the graphic card? I would be *very* interested to learn more about that.