The 80WPC rating is an RMS rating. Dynamic power is a synonym for peak power -- the available power supplied during short bursts (measured in milliseconds) without clipping. However, comparing RMS power is usually the best way to compare apples to apples.
Be that as it may, different manufacturers perform their ratings differently, and there's no real industry-wide accepted method of rating. Truth be told, the ratings are all obtained from monophonic sources into some sort of reference measurement equipment, and they may reflect very little how smoothly the receiver can drive the more complex load of a loudspeaker with polyphonic signal anyway. They're more a way of comparing different models from the same manufacturer, really.
Most listening is done at < 10W. Having an amp / AVR capable of supplying more power will get you more headroom and better dynamic range without clipping, so getting something with more power makes sense from that point of view. But then again, I'd guess a 75W Marantz would have about the same clean output as a 100W Yamaha, just for the reasons explained in the article that agarwalro linked.
Just a suggestion here. It might be more worth your while to suggest a budget and features you're interested in, and maybe we can suggest a model or two that'll match. Describing the dimensions of the room you're trying to fill couldn't hurt, either.
