It's taken me a while to build this system, but it really happened in two major steps. When I first bought the car (summer of 98), the factory tape deck was still in the dash and only the radio worked. It was actually kind of cool, I liked the feel of it and it didn't look bad, but I wanted CD capability because I only have about ten cassette tapes. The factory speakers were still in their places, as well, and all four of the cones were completely separated from the baskets. They buzzed horribly. So I immediately replaced the head unit with a basic Pioneer unit: one set of preamp outs, detachable face, and "Front Image Enhancer" which basically cuts out the highs going to the rear speakers. I also replaced the front speakers (6.5") with some Pioneer two-ways and I replaced the rears with Blaupunkt two ways (also 6.5"). By the way, the picture below says they're three-ways, but it's wrong. The Pioneers have excellent imaging and very good bass, and the Blaupunkts aren't as clear with the highs but they have excellent bass. I got all this stuff for a little under $400.
More recently (September of 99), I added tweeters for improved imaging and a subwoofer to supplement the low bass. The JL Audio 10" woofer came in a custom sealed box for about $200, and the amplifier and the tweeters came from Crutchfield for a total of about $180. So in total, the system has cost me around $800 (plus another $100 for the alarm), and it is really all I could want.
The bass isn't very loud outside the car when it is turned up all the way, but it's plenty loud inside (for me, anyway). I'm really glad I decided to get the amp I did; I originally thought 100 watts RMS would do the trick, but then Crutchfield talked me into getting a 180 watt amp and it is just right. I would totally recommend calling Crutchfield. They got some good deals going on over there. This amp cost me only $90...that's 50 cents per watt. Unless you have a connection with somebody, you may not be able to do better than buy from these folks. Plus they have an awesome catalog, a very comprehensive website ( www.crutchfield.com), and the phone people really know what they're talking about (I'm often underwhelmed at the knowledge of the salesmen in stores around town).
The tweeters actually don't make that much of a difference when I'm just driving around with the music in the background, but if I'm really paying attention to the music, the difference is there. If you're getting started designing your own system, I would consider tweeters only after everything else is in place, because I have to honestly say that they've given me the least bang-for-the-buck of my various car audio purchases. Good normal speakers are going to make the most difference in your system's sound, then a subwoofer, then tweets. Stick in your choice of head unit wherever you like.