I have the newest alpine single din in my truck, with components made by DD (digital designs) both front and rear. New deck will help alot, especially if you look into how much the deck actually puts out, the higher the output wattage from the head unit, the louder you can get, and ive found that a simple but high quality deck is a lot better then a fancy low quality flip out head unit. As far as a sub, i built a box that fills the rear middle seat, making an arm rest and storage area for the two rear seats, my box is L shaped so it bolts to the ground and wont slide around, i have a DD 9500 series 12 in it that can handle up to 6000 watts and makes an amazing sound. Tweeters are going to help the most in terms of clarity IMO, look into components, with the stock system your going to have to cut a hole for the tweeters in the door panel, but that was no big deal and an easy install. Quality is the main thing to look for when you are looking at speakers IMO, the better the quality, the better they will sound when you put more power to them, my door speakers are amped up by a kicker 450x4 amp, and there arent many trucks that will get louder then mine in terms of stereo.