It's a bit of an open ended question, really - do you want to know how to make XNA games? Then you should probably do a small project in XNA before you get started on this one (google for a few tutorials and make a snake game just to get a feel for the framework).
As for the actual rpg I think you might want to learn some scripting before you get started on something like that. It'll help you craft NPC interactions faster, for one thing.
Since you already made two games I'll spare you the whole "lower your expectations" speech but an rpg of any scale is still pretty ambitious in my opinion - if I was you I'd tackle something more along the lines of a tetris or mario clone.
Nobody likes working on something for a year then having to ditch it - completing 5 small games is better than abandoning 1 large one, if only for your morale.
Complexity will increase as your experience does, so don't sweat it