Is worth it to replace only front and rear speakers?

[seriousfun]

Speakers are by far the most important part of any audio system.

All else equal, an upgrade to better 6.5" component or coax speakers will be something you can hear immediately, within the limitiations of the head unit. Quart, Infinity, whatever, do some listening and choose what sounds good.

More power can't hurt - it will increase headroom, so you can play louder with less distortion. A subwoofer - even an under-the-seat Kenwood KSC-SW1 - will relieve the main speakers from some of their burdon and let them sound better.