Is your car bose equipped? Since you're looking at 5x7/6x8 speakers I'd assume not, but I gotta ask to give good answers.
Regardless, I would go with a good set of 6.5" round components in the front and skip doing anything to the rears. I have a few reasons to say this. First, the components will give you clearer sound than a set of coax's. This is mostly because they include a much more sophisticated crossover, but also the ability to choose a more ideal tweeter position helps as well. Secondly, the rear output from the stock head unit is crap. Hooking anything up to it is really a waste of time, energy and money. And finally, why bother upgrading speakers that will play limited bandwidth behind your head. Focus your money on the speakers that you will be hearing the most bandwidth from, the fronts.
In terms of choosing speakers. The best advice is to go listen to them. Keep in mind that listening to them on a soundboard is not the same as listening to them in a car, but it can give you an idea of the differences between different speakers.
