Check on the back of the speakers. It should say what the power output of the speakers are. I'm not a sound expert so I stand under correction but amps these days have y amount channels which can supply x amount of power which amount to a total capacity. So ie if you have 2 speakers with 80W power output each then you need a 160 (min) watt amplifier. A little more than the speaker power output is always good.
And make sure that the power is measured by using RMS voltages... Sounds like a nag, but it's true: PMPO means nothing and "watts rms" is a nonexistent quantity
And don't forget to check the impedance of the speakers (measured in ohms)