You need a wireless repeater. You put the transmitter inside (or above) your doorbell chime upstairs, then you plug the receiver (with chime) in an outlet downstairs. When your doorbell rings, the transmitter sends a signal to the receiver, which then also rings. I have a house just like yours (1600 sf bungalow) and we have one of these. It works flawlessly.
http://reviews.homedepot.com/1999/20...ws/reviews.htm