Car Speakers

I have a sony 4x45w rms cd/mp3 player. what speakers can i choose to go with the sony? can i choose 50w rms speakers or rms output of the speakers must be below the rms value of the sony? for example can i put 50w rms speakers on the front and 45w rms in the back?oh and one more thing.can i put speakers with built in tweeters in the back doors also?

Yes you can use 50w rms speakers, why not put an amp in if your putting 4 speakers in? i dont suggest putting another set of splits in the rear doors as it will create much tweeter noise, car audio is all about staging , you want the audio quality choose carefully where you place tweeters and your rear fill (rear speakers)