first thing first.....take all that sony xplod shit out of there.......its not worth the money for the child labor that made it
Second, forget about back speakers, at least for now. Pick up a nice set of 6.5" components for up front. Music is recorded for only 2 front channels (for the most part), so really front speakers is all you technically should have in your car. Rears can be added later, but should not be a main focus.
If you are looking to go cheap, look into Phoenix Gold rSD components, they can be had for less than $100 shipped. Also I believe there are some CDT components that can be had for cheap and have been getting good reviews.
Then get yourself a quality 2 channel amp that will give your components the proper power. rSD's will want at least 120 watts rms. Profile amps are great for the price. Millionbuy.com has them for the cheapest that I have seen
Properly install them and you will be happy, they will be far superior than the sony speakers you have