A good headunit is a good start. Now depending on your budget you could go new speakers all around or just a 10 or 12 inch subwoofer. Or even do a combo of both. The only time you would need an extra amp to run the extra speakers is if the hu isnt powerful enough. Most people can tell if the speakers are powered seperatly. If you have the money to do that go for it. Some good brands that I recommend are Alpine, Pioneer, Eclipse, Kenwood and Rockford Fosgate. All quality brands that will give you your money's worth and will last. Stay away from Panasonic, Audiobahn and Sony. Those are good brands with a lotta power but 1 year or 2 down the road...you won't have them anymore....good luck with the system and update us with what ya get. Oh and like saga said crutchfield is the best place to learn, wiring isnt that hard