Car Audio Sytem..?

I was planning on buying a system for my car but...I don't know what stuff I need to buy. I already got the Subs down (18" Audiobahns)...but what else do I need to buy?

I've been liking the AW12Q's I think they're called, by Audiobahn. They're 12" subs, fiber glass cone, rubber surround, 700W RMS... They sound really nice and go for a good price at 96.00 each. In my front I have two 6.5" Sony xPlode 4-way speakers (meaning it has the cone for bass + 3 tweeters), in the back I have Pioneer 6x9 4-ways (because Pioneer 6x9's are fucking awesome, I'm never changing them!!) and I have a HiFonics 400W RMS 2ch amp powering half my speakers on one channel, the other half on the other channel, and I am soon getting two Audiobahn 800-1000W RMS amps I haven't decided yet, that's the only thing I have left to buy. One for each sub. When you get your amp make damn sure it has circuit protection (ie blowout, feedback, etc) and one or two air intake intercooling fans to keep it from overheating. Oh yeah, also make sure you have at least 1/4 of an inch underneath your amp when you bolt it down to allow more air flow to keep it cooler. Be sure to keep the top and sides of it clear of any objects.

Cool, thx DFD. Lol, I used to think bigger = better...