Yes, you can use components speakers without additional amp.
Changing only the factory fitted speakers is the first step of improving the sound of car stereo.
A pair of decent (read Infinity, RF, Morel, Hertz etc) sensitive component speakers ( +/- a pair of coaxials at back) can improve the sound quality a lot, and is sufficient for many listeners. You will experience an improvement in clarity, mid and upper frequencies. Improvement in bass may or may not be there. So, audition before purchase.
Further to improve the sound quality, you may add:
1. 2/4 Channel amp. an amp with speaker level input will be easy to integrate
2. a sub. you may consider an under seat sub to save boot space. Though almost all experts here will urge you to opt for a regular sized sub for better sound quality.