Audio Systems: Adding more speakers, ohm load, radio shack

Question
I have Onkyo TX-8522, 100Watts/channel, and I would like to add two more sets of speakers.  I am confused about how to do this as to whether I need some sort of preamp or what.  I know I can't just add them to the existing system without doing something.  If you can recommend a proper piece of equipment for my needs, that would be appreciated.  I'm no audiophile, just want to hear my music all over the house.

Answer
You must protect the Onkyo. It wants to see about an 8 ohm load. In order to do this you need to purchase some sort of impedance matching device like a speaker switch (Niles, Sonance, Radio Shack) or impedance matching volume controls. These are transformers whose primary will always make the Onkyo think that there is a constant 8 ohm load no matter how many speakers you add (of course within reason).