Here you can find the distribution of Android platform versions. It will need some times until a large part of the users are using 3.0. So I would recommend to focus on Android 2.1 and higher if you have to specify. If you develop for 3.0 only, no one will be able to use your application. That can't really be your goal, right?
What exactly are you're referring when you mentioned "Graphical Layout"? The visual GUI editor? If that, I would recommend to do your layout in the XML file and then you could check how it looks in the "Graphical Layout".
