I would be very relunctant to buy speakers without first hearing them. Different people prefer different types of sound. In addition, the type of music, movies, games etc. you put through the system will make a huge difference. The Z-680's seem to be geared towards gaming and movies becuase they certainly add a fair bit of low end punch.
Before you buy the NSP-1's, you should definitely listen to them - preferably side-by-side with the Z-680's to hear the difference in sound.
Hooking up the Z-680's to another receiver will work - but you will have to be careful with make sure the volume doesn't get cranked to high on the Sony receiver.
Finally, as I mentioned above, you may be able to work some of your peripherals directly to the Z-680 system, thus bypassing the receiver. The DVD player will definitely have a digital out. Many PC's also have digital output. Some satellite and cable tv's also output a digital signal. If you have a bunch of devices that have digital audio output (TOSLINK or Coaxial), they you can hook them to a switchbox that connects to the Z-680 receiver.