Its hard to tell because it depends on several different factors including, but not limited to, the speakers your using, the frequency that the speaker is reproducing, the output voltage of the preamp, some preamps maximum output voltage is 1.0V and your amp may require 1.6V for maximum rated output. It's always better to have more power on hand than needed. This way you can virtually eliminate the possibility of amplifier clipping which will cause speaker failure. Also if the amp has plenty of headroom it will sound more dynamic and detailed because it's nowhere near it's limits.
I think it is more accurate to say that many other receiver manufacturers (particularly in the under $1,000 range) exaggerate their power ratings. The FCC doesn't seem to be very strict in this area and a lot of receivers seem to only drive 2 channels on their 5 or 7 channel receivers, yet they give a 7 x 100 watt specification, when often if they drive all 7 channels to clipping they will get anywhere from 35 watts to 80 watts.
Sound and Vision is one of the few publications that seem to take accurate measurements (most of the time)of commercial receivers under $1,000 and show the graphs accordingly. Stereophile usually just tests expensive receivers and separates.
Most other publications don't seem to publish any graphs from their test equipment (if they have test equipment) and just trust the manufacturer or just give you their subjective impression.
Again, Hawk was kind enough to give a link to a Sound and Vision testing of various receivers in the years 1999-2002 a few months back. It was quite eye-opening. There weren't many receivers that had the same 5-channel or 7-channel amplification as was listed on the companies spec.
Normally the companies high end receivers met or were very close to meeting the specs on power. I remember the NAD T752 listed at 80 watts by the company tested out at 92 watts x 5 channels, which was very good. At the time Pioneer Elite's top of the line 49TXi which the manufacturer listed at 130 watts actually tested at 148 watts into 5 channels and 144 watts into 6 channels. Then it was a bit shocking to see receivers like a Marantz SR7200 that the manufacturer listed at 105 watts x 6 channels tested at 27 watts x 6 channels. Whhew. The HK receivers uniformly tested very well.
The reality of loudness though is that the difference between a true 100 watt receiver and a true 80 watt receiver is mostly undetectable.
The other point to add to Gregory's is look for the word "continous" in the power specification. A short burst of power at so many Watts is meaningless, but that is what many manufacturers give you, without saying so.
To Shaun, sorry you are wrong. Though not a complete answer an amplifier can basically deliver x watts to any frequency within it's stated frequency response which is generally specified, as directed by the FCC, to be 20 - 20kHz. There are some variances to this but have to be stated on the spec sheet and any numbers given as -3dB or so are there more for bragging rights. What you do not want is an amp that churns out 125 watts at 1kHz, 75 wats at 20kHz and 10 watts at 20Hz. There are specific rules for how an amplifier is measured that include load, warm up, 1/3 power, etc. You can find theese by searching the web, try Ask Jeeves. The ideal amplifier not running through an output transformer, which is most solid state (transistor) amplifiers, should double it's voltage swing every time you 1/2 the load impedance and 1/2 it's voltage as load impedance doulbles. That means, ideally a 50 watt amp at 8 Ohms will produce 100 watts at 4 Ohms and 25 watts at 16 Ohms (gosh, I love my ls35a's). This is seldom the case in real life because most amps do not have the power supply to handle this and at the hi end where you pay for the power supply the trend has become to have a very stiff power supply that is technically capable of that type of performance but is designed to be more stable into a reactive load of a real loudspeaker as opposed to the resistive load of a test bench. An amplifier will produce it's maximum wattage if the input is sufficient to drive the amp. If the amp is spec at 2 volts in then it requires 2 volts to reach full power. If your input voltage is 3 volts the amp will develop full power earlier on the volume dial assuming you do not overdrive the front end and clip the driver section. If, on the other hand your input can only produce 1 volt your power amp will never reach full power. It certainly is not true that more power is always better. I would refer you to my answer to "Harman Kardon" and then suggest you go to a hi end dealer and ask to listen to a really good amp rated under 50 watts or if you can find one a single ended tube amp that maxes out at 8 watts.