You need lots of vertical space, so don't get a widescreen monitor. Since you are not doing graphics work but only design work, I don't think you need accurate color reproduction, reasonably good color reproduction will be enough for the task.
Dell ultrasharp 20" would be the best option, if you can get one (by luck). Otherwise, viewsonic or samsung lcds having aspect ratio 4:3 or 5:4 are the only options you have within your budget.
If you buy a widescreen lcd, then ensure that it has pivot ability, that means you can rotate the screen and increase the vertical space.