Your head unit says 50w X 4. I can only be lazy and guess that is peak. What components do you have in the front right now? This sounds like you WILL NEED AT LEAST one amp. If you are going to add a subwoofer, you will need an amp, and depending on what you want for your setup, if you amp any speakers, the rest will be drowned out if you don't amp the rest. As of now, your components might work, but I'm sure they sound bad if you try to turn it up on such low power. More people blow speakers on too little power than too much.
Find out what model your components are their wattage ratings for both sets of front components (RMS and peak). Not sure if its the same components that you have for the rear, but if the RMS ratings are close enough you could find a 4 channel amp, and run all four component sets off that with the better ones up front (not necessarily more power).. Depending on your subwoofer, you then might be able to get a 5ch amp if the subwoofer is on the smaller end of power, and you can find something for your needs.
Either way, Once you start adding amps, you need to make sure your electrical is up to par. You will want the big 3, a larger alternator, and IMO figure out how big you MIGHT want your system to be.. You can always just include larger wires for easy upgrading later, but its a kick in the ass to be one size smaller because you wanted to upgrade the amp or speakers later then have to add extra time and money when it was only a couple bucks more if you planned ahead in the beginning.
