The x1300 chipset certainly supports Dual-Link DVI (the vital issue here) - but if your card's manufacturer chose to connect all the pins to enable this functionality is another issue [img]/images/graemlins/frown.gif[/img]
Looks like your cables are all fine, so I'm starting to lean to the fact that they didn't bother [img]/images/graemlins/frown.gif[/img]
At a guess, I'd think your card is the one halfway down
this page - unfortuantely there is no part # so I can't figure for certain if it can do Dual-Link... but your experiences suggest not at this point.
EDIT: Scratch that, ther eis a part # - it is probably a "JJ461" - but the fact that they mention Dual-Link for some of the NVIDIA cards and for the ATi FireGL, but not for yours is not looking good imo.
Gonna guess you need a new card [img]/images/graemlins/frown.gif[/img]