dear LM and arun,
speakers are Very forgiving, so having an amp/speakers even with 50-200% range of each other should work. the benefit of having a higher capacity speakers hooked to a slightly smaller amps 90w speaker on 50w ) is that speakers will have more headroom. that will ease their stress and produce cleaner sound rather than having 90w speaker do 90w.
LM, top speed of your car might be 150 kmph, but do you always drive so? (excluding traffic conditions ) no, you usually drive around 60-90 kmph, keeping some headroom for capabilities of the car as well as getting a better fuel efficiency.
arun,
you may shop for speakers freely without worrying much about wattage. look for stiff cones and high roll surrounds. a large magnet also helps. as your amp is small, take a good look at sensitivity number. somewhere about 90db/watt is better for lower powered amps.
this number is like fuel efficiency. playing louder with less wattage.