Upgrading Honda Civic 02 Speakers?

I have an 02 Honda Civic with stock speakers and a recently upgraded head unit (Pioneer MVH-x640bt), and I want to know if I can upgrade the two front speakers without an amp? Even with the new HU would I be losing so much quality that it wouldn't be worth it or would I see some quality increase?

Added (1). The model is MVH-x560bt*

You would most definitely be upgrading your sound quality. Though the factory speakers in your Honda are adequate they are really nothing special. Even an averaged priced Infinity or Polk Audio speaker would make a very nice quality upgrade.

I believe your car is practically the same with my civic 2003 EX.
It already has factory installed 6 1/2" speakers - doors & rear.
No matter what size you decide to install (6", 6 1/2, 6 1/4 or 5 1/4 but I'd suggest 6" to 6" 1/2) you'll need a special adapter/spacer for civic (check ebay or your local market) which looks like this
http://www.ebay.com/...0710579661
and of course speakers not deeper than 2 1/4" should be used because speakers with extreme depth don't let windows coming all the way down.
In response to your question, I can't tell you which brand will be best for your ears as different brands cover different musical preferences so I think you should make a research in your friends' installations and local stores, listen to some music and decide what satisfies you most.
I can only say that you'll have a major improvement if you make a good choice so keep in mind that stock speakers (front and rear) are very sensitive (93 dB) so you'd better consider efficiency when buying new ones (prefer sensitivity of 92dB to 93dB) in order to avoid disappointment.
It would be also useful to see on youtube how to remove-install door panels.