matt hit it right, when it comes to wattage, MAX watts may get your attention but RMS is where you want to look. RMS is the average power or mean power.
RMS is typically what people like to go by.. normally, in the audio industry, the more money you tend to spend, the better the quality. (at least in my eyes)
go with reputable brand for speakers like JL, kicker, Infinity, boston acoustics (i love them).. just some good name brands.
good brands that i know of for decks are alpine, kenwood, clarion, and panasonic.. im sure i forgot a few other good ones but off the top of my head.
also, dont go with a speaker that can produce sounds way higher than 20,000 hz. the human ear can't pick up anything over 20,000 hz. i know that alot of them go up to like 22,000 and stuff which is fine but i wouldn't pay the extra money for speakers that go up to like 30,000 hz. someone correct me if im wrong because my logic seems correct. i may just not understand why? is there a benefit to this?