Resolutions above 'HD' (1920x1080 or 1920x1200) require you use a 'Dual Link' DVI cable. If the cable you had was single link (google those terms to find out how to ID the cable) then it will limit the resolution, or look like hell.
You need to (as you did) use a displayport cable if your monitor supports that, or get a Dual Link DVI cable (would be surprised if the monitor did not come with one)
Note also that if you are connecting to something like a Mac, using their mini-display port or
thunderbolt connector, that most of the cheap passive mini display port to DVI adaptors are Single Link, and thus will not work at the higher resolutions. You can get mini-dp to DualLink DVI adaptors, but they are not cheap, ones from places like Monoprice.com run for around $50, and the official Apple one is likely close to double that. If the monitor supports displayport, just get a mini to regular DP cable, it will be a lot cheaper. The only reason to go with a mini-DP to DualLinkDVI adaptor would be if the monitor does not have displayport, or you are doing something like feeding a DVI KVM (Keyboard, Video, Mouse) switch to serve two systems with a single display. (in which case it better be a good KVM that supports Dual Link DVI