Just to add about briding - I reccomended the 4 channel amp bridged because you can get more power on a tighter budget. It does come at a cost, it effects the damping, but unless your talking about competing in SQ events, then as a average listener you don't need to worry.
Bridging is basically taking 2 channels, and combining them to form one more powerful channel. It relates to what acarmody said about impedance levels. You will see listed on an amp specifiaction sheet, 2, or 4 ohm power outputs on most multi channel amps. It can be a little difficult to explain, but i'll give it a go. Acarmody's idea about just googling it is sometimes the best way to go about it, you might find you understand some explanations better than others.
You have 4 channels, they each have a base impedance of 4 ohm. If you bridge both the front channels together, and then the rear, you've now gone from 4 down to 2. What is happening is it's actually making each individual channel operate at a lower impedance, or half it's impedance, in this case 2 ohm. Pairing each of those channels together, now brings the impedance level back up to 4 ohm, through half the amount of channels.
Now i really find it hard to reccomend ampifying rear speakers, let alone spending decent money on replacing them. I could go on for ages as to the reasons why, but instead i'll try and just find another thread covering it and link you to that so you can have a read through it.
