Your 7800 GT probably has DVI-I outputs rather than DVI-D. Almost all video cards except very low-end cards have DVI-I outputs rather than DVI-D. In any case, you only need a single link DVI-D cable to run at 1440 X 900 but a dual link DVI-D cable will work fine too. I'd get a dual link cable anyway because they barely cost more than single link cables and will be more useful in the future at high resolutions. Dual link DVI-D cables are the "standard" DVI cables these days. They're compatible with both single and dual link hardware.
This page gives the gory details on DVI compatibility if you'd like more info.