Every system is different and very subjective but I'll give you my opinion as I'm doing a similar build right now.
If you are after best possible sound don't bother upgrading rear speakers, put that money towards a processor. You want to build the best front stage you can. Keep stock rear speakers in and fade to back for a little fill only when you have passengers back there and the music is too quiet for them.
Eliminate the dash speakers as they are full range signal running to a very small speaker so the mids to low end distort quick as you turn them up. You also end up dealing with sound reflections off the glass because of how they sit.
Without a DSP you would have to run a passive system using crossovers so there's less control there than if you run them active with a DSP controlling it all. You also get a lot of other fun controls to play with like Time Dilation and individual speaker EQ'S.
This is of course just my opinion and I'm sure you'll get a few
I am running (or will be once they are in tonight) a set of Morel Components and 2 Image Dynamics 10" subs with control coming from a Bit Ten Processor.
I have run a good set of components and 2 10s in a 2004 Ram QC and there was more than enough sound to fill that cab and sound great.