There's nothing wrong with building over time. Indeed, it's fun to add those pieces and watch your favorite movies all over again. A 2.0 setup with competent towers can be enjoyable for movies and excellent for music. Add a competent sub and you're now digging deeper with more authority while freeing up your receiver to feed a bit more power in the higher frequencies to your old speakers. Suddenly explosions and crashes and special effects of all sorts can take on a new dimension. Add surrounds and movies come even more to life. Don't forget a center channel offloading voice duties and firming up the soundstage.
The order is arbitrary, of course, and subject to your preferences, desires, and the unbeatable sale you come across.
See the Denon xx13 thread for all you want to know (and probably a lot more) about the 1713 and other models. Look at the features you need and want and decide based on that. Just educate yourself a bit first about features you may not really understand yet, like Audyssey.