DVI resolutions above 1920x1200 require dual link DVI. (There are exceptions to this depending on the refresh rate, but thus far the two consumer monitors out there need dual link).

This is still a single DVI connector, however it feeds two separate DVI signals over the cable.

Searching for Dual Link DVI on NewEgg comes back with 9 cards. The key part is "Single and dual link external TMDS transmitter support for high resolution and/or multi-monitor DVI configurations"