Do the rear speakers need to be amplified and plugged into the wall separately in order for the system to work? A Best Buy employee at the store said that my rear speakers have to be powered speakers.

Not necessarily, my rear speakers work just fine with my wireless speaker kit. The only thing you need to do is plug the rear speakers into the wireless receiver kit which is plugged into a wall outlet. The kit sends a wireless signal to your stereo receiver and then you are ready to enjoy surround sound without those ugly speaker wires. This is probably not the best technical answer you were looking for, but it is working great for me.