I would say, since you want to do 2D, the tutorials here might not be helpful. (If you want to do 3D, then they certainly will be very helpful)
Since you have already done the 2D tutorial, ensure you take a look at and gather the concepts in the Game State management sample, and then I would plan out the game first. Once you know what your game is going to be exactly, you can go and find appropriate samples and tutorials at sites like Ziggyware (which Mike mentioned) and if you have any further issues, you can always ask here or on the Creators Club forums.
James Silva is a hobbyist game developer (well was, since he has the XBLA contract now) just like most if not all of us here, so its certainly possible with time, just keep learning and applying what you have learned to games along the way.
The usual tips are to learn by creating a clone of pong or tetris, or any of those simpler games, and just choose something harder as you develop your skills, then you have practical experience and can experiment with your ideas safely.

Good Luck!