In order for one amplifier to reproduce sound twice as loud as another at any given volume setting, you need 10 times more wattage output. An amplifier rated at 100 WPC is capable of twice the volume level of a 10 WPC amp, an amplifier rated at 100 watts per channel needs to be 1,000 watts per channel to be twice as loud. In other words, the relationship between volume and wattage output is logarithmic rather than linear.
The quality of the amplifier is as (or more) important than just the wattage output. An amplifier that exhibits excessive noise or distortion at loud volume levels can be unlistenable. You are better off with an amplifier of modest power output with a low distortion level that a much more powerful amplifier with a high distortion level.
When shopping for an receiver/amplifier, be wary of wattage output specifications.
For a more detailed explanation of factors that you need to take into consideration when choosing a good Amplifier or Home Theater Receiver, check out my reference articles: