the 1:1 option shows the picture as it is, it doesnt upscale or stretch it (or at least it should work like that) ending up with black bars and clearer picture when lower than native res is used.
not sure exactly how the wide and zoom works on that monitor. probably has something to do with how the monitor handles non 16:10 aspect ratio resolutions. in a any case you should always use resolutions with the same aspect ratio than the native res, otherwise it might look a bit distorted.
16:10 (or 8:5)aspect ratio is marked with purple in this charthttp://en.wikipedia.org/wiki/File:Vector_Video_Standards2.svg
so smaller ones would be 1280x800 and 1440x900