Power ratings on speakers do not equal louder speakers; a higher rated speaker should be able to handle a louder signal however (if you could supply it with enough clean power). A highly rated speaker will be able to sing at party-levels for a long time without overheating its critical components. Whether you really want that is another story - most people really don't need it.
Power ratings on receivers & amps need to be carefully interpreted - not all ratings are the same. Look at what bandwidth they're rated at and the THD figure. Some manufacturers publish specs with for fixed frequency signal with high THD (distortion) which makes them look good. Watch out for this.
Keep in mind that for every 3dB increase in SPL, you need to double the power. So to make a small audible difference over a 100w/ch amp, you might have to go to 150-200w/ch - which equals a beeeg price difference. On the other hand, the difference from 80 to 140 watts which is where most receivers lie won't really be noticeable; other things will influence the sound much more. Signal-to-noise ratio, damping factor, quality of power supply, supported formats, inputs & outputs are all very important considerations. I would go as far as to say that any amp/receiver from a decent brand in the mid-range upwards will give you what you need. If the receiver has pre-outs, you can always add a dedicated power-amp later.
Typical listening volumes (even loudish ones) only require power in the region of 10-20w/channel. Any receiver can provide that, but you need the headroom to accomodate peaks in the signal (explosions, dramatic moments in music etc.).
Bottom line: No, if you buy a speaker that says it handles 200w, you don't need a 200w amp to make it sound good. Look at its efficiency rather: that tells you how many watts it takes to produce X amount of SPL (i.e. loudness). Higher efficiency = less load on the amp = generally better.
Hope this helps
Go out and listen to different set-ups. The speakers & room acoustics make the most difference. Read the Audioholics "Get an AV Education" section for more - you will find everything there is to know there
(ok, not quite, but everything you really need and then some...)