I agree with JoshC. Most of the good aftermarket speakers do not sound well until you give them some good amount of power. I can hear the difference even when a cheap $50-60 coaxial speaker set is moving from head unit power to an amplifier channel. Typically 50 to 80watts per side is pretty decent power, although some speakers could take over 100watts.
Regarding speakers, I see two options. One for the lazy and another for the more creative.
The lazy option: put a 2-way component speaker set with the 6.5 speaker in the door and tweeter in the dash.. use head unit power until you have an amplifier for these. There are plenty of threads about budget 2-way components here.
The creative option: if you car fits 4-inch coaxial speakers on the dash.. this could result in a very interesting setup.
Put a 4-inch coaxial speaker in the dash board location and let it play the entire spectrum down to like 200Hz. Use the door speaker strictly for mid bass duty, with a low pass crossover frequency of about 200Hz, and high pass frequency of around 60-80Hz. Ideally you need an amplifier with a bandpass capability. One amplifier that could work is PPI P900.4 because it has bandpass capability for at least one set of speakers. You could use two channels for the 4-inch coaxial speakers with high pass filter, and the other two channels for mid bass speakers with band pass settings I described. Regarding the rear speakers, put there whatever you like, use the stereo deck power and attenuate them. They're not very important.
For the door midbass drivers, you could just use Morel Maximo or Alpine Type-R coaxial speakers. They have reasonable mounting depth and are cheap. The woofers have good mid bass response. The tweeters... who cares.. the driver will be bandpassed.
The options for the 4-inch coaxial speaker are extremely limited but a couple of decent options exist, JBL P462 or Morel Maximo 4c. Both are sold under $100. If you can afford to spend more, HAT should release the 4 inch Mirus coaxial speaker some time soon (only 6 and 5 inch speaker is available on 12velectronics last time I checked, but the manual mentions 4 inch speaker as well). Why would this setup sound better than a conventional 2-way speaker? The conventional 2-way has a crossover frequency in the upper midrange, often screwing up imaging if the woofer and tweeter are placed far apart. With a single 4-inch coaxial speaker acting as a point source, you don't have this problem. If you have the ability to time align the left and right 4-inch speaker, this should result in an amazing imaging IMO.
