If you want good quality sound, focus on the front speakers the most. If you can afford it, go components/seperates... then just get a set of coaxials(ie 2-way or 3-way) for the rears to fill in the space. If you do get components for the front, you will want to get an amplifier to get the most out of the speakers.
Just remember that a round speaker will always reproduce sound more accurately than an oval speaker(ie 4x6"). If you want to make the front adaptors for a 6.5" speaker, here's a template:
http://jamie.hackthebox.org/fi...e.pdf
You'll need Adobe Acrobat to view/print it(just make sure to turn OFF scaling when printing: it will fit on an 8.5x11" sheet of paper, use the ruler to verify). Anyways, when people say re-wiring, they are likely talking about the wire going to the speakers themselves. This is relatively easy for the rear speakers, but trying to run wires through a door jamp harness is a huge pain... not worth it in my opinion for non-competition systems.
All-in-all, if you are on a budget, simply a new set of factory-sized speakers will sound a *heck* of a lot better than the originals. It's all in what you want to spend.
To give you an idea, a decent set of front 4x6" coaxials shouldn't run you more than $50-60. A set of decent coaxials will run around $150+, plus the cost of an amp+wiring.
HTH, and it wasn't too much to digest at once!