ur monitor has the optimal resolution at 1440*900 which is specified by the manufacturer. There are some monitors 19" which can also use 1680*1050 too, which has somewhat 2 native resolutions.
btw ur card has the max res of 2560*1920, but it ll be limited as per the type of monitor u are usin.
usually for CRTs, there are no optimal resolution thing, so u can go much higher if u can bear the low refresh rates. And LCDS luks much worse on non-native resolutions higher or lower.