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.