Heya Nex! Saw that signature, and had to re-check what forum I was reading, heh //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
The speakers you have now are coaxials. The tweeter is mounted coaxially with the woofer. In a set of components, they are mounted separately, and have an external crossover that controls what frequencies go to which component.
I just ordered a set of Rainbow SLC components from Don (6spdcoupe). Midbass driver will go in the door speaker location, and tweeter will be 'glassed into the sail panel on the inside of the mirror.
Depending on how you like your car to sound, you could just keep the 6x9's you have now in the rear deck. As has been said before, just set the gain carefully, your 300/4 has more power available than those speakers can take.
A set of components up front would give you the best possible sound quality. That should be your first order of business. Once those are installed, then if you feel like you want more sound from the rear, upgrade the 6x9's next, maybe Pioneer REV series or something.