While the appropriate answer is ultimately based on the recommended amplification range of your speakers, 50W-100W per channel is typically more than enough to satisfy the vast majority of surround sound speakers (75W being the sweet spot in a 5.1 or 7.1 system). That's not to say there aren't speakers that require a higher RMS rating, but if that's the kind you're looking at, you probably know you'll need to look at higher-end AV receivers or even discrete amplification (maybe you're even working with an integrator).