Aftermarket speakers sound great, but you don't need an aftermarket amplifier to run them. Amplifiers do what amplifiers do best: make things louder! Unless you plan on turning them up loud, I wouldn't spend the money.
Nobodys said anything about the speakers. Bouston Acoustics makes great computer speakers, I know that. I haven't tested any of their mobile audio products. I would go check them out at your nearest electronic store. Test them and make sure you like them. If they sound like crap at the store, it will be 3 times as worse in your car. Choose wisely.