If you get a set of 4x6 speakers for the dashboard, make sure they have a gasket on the speaker's face, becasue they flush mount to the dash.
SO many options for rear speakers. If you get a set of decent size 6 1/2",6x9, etc. you might want to put a set of high-pass filters ("Bass BLockers") on the front 4x6's. They don't handle bass well due to their size and the fact that they are not in a sealed enclosure. You can get a set of those for under $10.