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 )