well you could buy any receiver like this.. http://www.bestbuy.com/site/olspage.jsp?skuId=9245836&type=product&id=1218067378669 for 2 speakers only
or
if there is 5 speakers then you will need a 5.1 channel Receiver like this
http://www.bestbuy.com/site/olspage.jsp?skuId=9245523&type=product&id=1218066769427
or
if there is 7 speakers then you will need a 7.1 channel Receiver like this
http://www.bestbuy.com/site/olspage.jsp?skuId=9245854&type=product&id=1218067378840
or
if there is more speakers in different rooms then you will need a mutli-room receiver
all those receivers are just examples but you might want to look into buying something else other then a sony.. maybe something like a denon, yamaha, or karmon hardon receiver but before you go out and buy a surround sound amplifier i would go pull out one of the speakers to check the wattage out on them. so a store can help you out picking a receiver that's right for you and what you need
if there is speakers in the wall you would also need a subwoofer i would recommend a polk powered subwoofer or even a klipsch or a martinlogan powered subwoofer.. without a powered subwoofer those speakers won't sound right it will just be mostly all mid-range and treble no bass at all (not made for bass)... hope i helped..