Speakers have different efficiencies (loudness) and the smoothest response is often not as loud/watt as more average sounding speakers. If you can simply adjust the amp fader to balance your speakers front to back you may be very pleased with them. If not, get the rated efficiency of your current speakers and find some dash speakers that both fit and match the rest of your system.
I personally like Polk, Boston Acoustic, Focal, and Alpine brands but finding a good match for your system is often more important than buying the "bees knees". Crutchfield is a good resource.
Car speakers: what to look for