how to know what power to get-match speakers best?

There is no such thing as having too much power.
The biggest danger is not having enough power; you may end up running your amp too hard and clipping occurs and speaker damage occurs.
Your second question: Doubling the wattage increases volume by 3 db.

My speakers sensativity is 93 db/watt. They are capable of 120 db at 3 meters. I have 1200 watts per channel. Will I ever use all that power? ... NEVER! ... but my sound is wonderfully clean, and there is no chance that I could ever drive the amps to clipping.
Richard

Cannot agree more with Mr. Richard; you can (hardly ever) have too much power. My speakers are rated 100 watts but I think they sound best with at least a 200 watt amp. One of my amps, at only 100w/ch, is simply inadequate powerwise although it is a great & well respected piece. Another amp has 150w/ch & that is still not enough. Another one is rated conservatively 180w/ch & that one works great as it actually delivers about 225. Anothere one, rated 350w/ch, actually delivers more like 550w/ch & that one absolutely never runs out of steam regardless of drive level. I took it to the extreme one night, & was afraid that the speakers were going to come apart if I pushed it any further although I was nowhere near max. But with all of that clean & unclipped power there was no fear of frying a voicecoil with squarewave distortion, although the drive level was WAY over their rated handling capacity. Too small an amp, pushed too hard, & a blown speaker is practically a given.

ok
all my life i thought that overpowering amps killed speakers.. i then hear about something called distortion and i thought that this was killing speakers..
now u both say that too little power can kill both speakers and amps ?

euh ok..
how come ?
and how can u push 100W rated speakers with 500W amps and not blown the speakers ?

thanks :)
Jin who needs to read more about audio engineering ( can't wait to get my books delivered from amazon ehhe )

i dont beleive watts are related to db directly(i beleive db is actually a sound pressure rating)-i think youre speakers specs have more to do with db rating.watts are related to volume because as you increase the volume youre amp has to deliver far more watts(double per increase i beleive)-this is why most receivers and many amps start to produce audible distortion and sometimes clipping when pushed(sometimes just moderately pushed)-and this is why amplifier wattage rating means almost nothing when your talking about most mass market products and many high end products because it doesnt tell you how well the amp can deliver the power as the volume increases.an amperage capability is a far more useful as in the case of harman kardon and nad products which can deliver beetween 20 and 30 amps per channel(vs. probably less than 10 for most recceivers and amps)-amperage is a rating of power as in your home outlet delivers 15 amps and all your electronics list on their backs how many amps they use-this does not mean you need massive amps to make a good amp or receiver though-its really more about good design.if you only need to play music at low to med volume all of the above may be irrelevant but if you need to turn it loud occassionaly i consider the above very important-heres my reccommendations for power delivery based on my admitedly somewhat limited experience--mass market products-hk,nad,yamaha,onkyo,outlaw audio.mid-high end--nad,roksan,chord,calaudio.i highly reccommend the roksan kandy or caspian over the mass market products unless you can find them at about half retail or if your budgets limited or if you need a home theater(vs.stereo)receiver or amp.

I've had it explained to me this way (and I could be wrong so if someone could verify this): If an amp, of any wattage rating, has the capability to generate double its wattage rating into successively smaller ohm loads (200 watts into 8 ohms, 400 watts into 4 ohms, 800 watts into 2 ohms) it means that the amp has the ability to deliver the maximum amount of current needed to drive the speakers at a given volume. An amp with higher wattage into 8 ohms, say 250 watts, that can successfully "double down" its power rating into lower ohm loads, can push the speakers to a higher volume level without distorting than say a similar quality amp with only 120 watts into 8 ohms. The end result will be a higher decibel rating in the room, say 106 db over 100 db, without distortion.

The speaker rating for wattage, say 50-200 watts into 8 ohms, is to tell you that with a certain wattage amp the speakers can theoretically be driven to a certain db level without damage. If the amp cannot provide the necessary power as the impedance of the speaker changes during musical transients or as you raise the volume level, the amp will "run out of punch" and clip because it does not have the power to push the current the speakers need. This is bad because the "clipped" waveform will eventually fry your speakers. If the amp is "too powerful" (is there such a thing?), meaning the amp is rated higher than the wattage rating of the speakers, then the opposite can happen. The amp has the ability to push more current into the speakers in order to generate higher volumes and to react to dropping impedance loads. At low volumes this effect manifests itself as "bass control, increased soundstage" and the ever-searched-for "air". At high volumes (And I mean ear-splitting volumes), you can blow (or melt the voice coil if it heats up) the speaker because you have exceeded the transducer's ability to change electrical current into physical motion. The rule of thumb that I go by: always keep your weakest link moving downstream. With a powerful amp to tame your speakers you can be sure that most of the information is getting through. Plus you go easy on your electronics (was your car optimized to run at the redline or at about 2500 rpm?). In your case, seek out a really good 250 wpc amp and you will be very happy.

Adding more power does not add an equal amount of volume. It is a factor of 10. To get an amp that will play twice as loud as a 50 watt amp, you would need 500 watts.

Add to the above good comments most speakers are designed to handle transents (short term power peaks) well above the max rated power some as much as 10 times rated max. I agree with you can never have to much power buy the biggest amp you can afford.