Nearly all (if not all) ATI/AMD and Nvidia cards has supported resolutions up to 2560x1600 (newer cards above that) for over a decade now and some cards older then a decade supported 2048x1536 so it's safe to say his card supports the resolution and he do not need a dual link cable for that resolution.
My bet is drivers on or something else on AmigaOS 4.x :/
