MP3Car.com forums are your friend. TONS of info there. Been lurking for about a year and a half and I'm getting ready to start my build, it's my xmas present to myself.
http://www.mp3car.com/vbulletin/
O'Reilly makes a decent book on the subject. Not necessarily the greatest resource, but for $15, its decently informative.
Some things I would talk about ruling out right now though:
Security system - your battery will NOT like you. You're going to have the pc running almost the entire time. There's a guy on the MP3Car forum named shotgunefx, he has a security vcr in his trunk, you may want to talk to him about that.
Vehicle control - depending on your software skills (I don't know enough to comment) its possible, it also depends on the kind of car you have.
Some general thoughts: There are schematics out there for DIY carpc power supplies that you can hardwire onto your car's 12V line, you can go power inverter, many options. Suggest you find out what kind of amperage your alternator can supply. With a 150W opus, you're drawing almost another 13 amps, make sure you have the space to play with. Being in Texas, you have to give a very long thought to cooling. I'm in the same boat as about 3 months a year, its too cold around here for a HDD to spin up safely until the car warms up. You're looking at frying your CPU unless you look at something to keep the PC from automatically booting until the AC takes over.
Joe: is it possible to move this thread to say Random Nonsense?
What kind of car are you planning the install in?