For people going from 2 channel to home theater, I recommend adding speakers in this order:
1- subwoofer. for music you don't really need to hear anything below 40hz, but most hollywood movies have a lot of special effects sound below 40 hz. a sub will complete the audio spectrum for your movies and give you the impact of a big theater. A good 2 channel system with a sub sounds really good with dvd's.
2. now you need a dolby/dts receiver. Don't spend too much if you already have a good stereo amp. use your stereo amp to drive your main speakers and the receiver to drive the other speakers.
3. surrounds. there's no point having the receiver without getting surround speakers. throw some qs4's or 8's behind you. Going form a 2.1 setup to 4.1 makes a big difference.
4. last but not least, add the center. If you get a center, it becomes the most important speaker in your system so get a good one. Audition center speakers until you find one that sounds more clear with dialoge than your two main speakers. oh, make sure it blends well with the mains too. When listenting from the 'sweet spot' a center speaker makes a movie sound different, not necessarily much better, but what the center speaker does is anchor the dialoge to the screen no matter where your are sitting.
good luck!
