Stereo speakers to computer speakers?

I have two speakers that came from a stereo system that broke. They both have a max handling capacity of 5 watts, an impedence of 4 ohms, and the imput wires are simply positive/negative wires. Is that wattage additive or constant when concerned with building an amp? What type/size of transformer do I need to utilize? And after purchasing the necessary parts (please let me know if I miss any necessary components), specifically where in the circuit do I incorporate the various components? I hope this question isn't too taxing...