I'm guessing that a 20.1-inch or true 21-inch monitor should support the native 1600x1200 resolution. Just make sure that when you are checking out the monitors online, or in store, that the specs state that the NATIVE resolution is 1600x1200, otherwise at that resolution, you will lose a little bit of image quality. Not much, but enough for an anal person to notice.
Honestly, the 19" monitors cost a lot less and look just as good at 1280x960. But if money is no object, go for the 20" or 21".