The Kappas will need an amp for best results. I doubt that you'll find them loud enough without an amp, unless you have a very quiet car, and never drive with your windows open.
More sensitive speakers, such as the Infinity Reference series and JBL GTO series are designed to sound good without an external amp.
I know that Crutchfield has sensitivity specs online for all the speakers. That's an easy site to leech good info from, even if you don't want to buy from them.
Don't forget the wiring harness (REQUIRED, or else you're asking--nay--screaming for electical trouble), and dashboard fit-kit (required for many cars).