I simple serach in google would give you the answers to your 7.1 question.
http://www.dolby.com/consumer/....html
However if your question is specific to what does this mean in the car, then the answer is basicaly, nothing. the stage inside the the car is so small that 5.1 vs 6.1, 7.1 (or very soon 8.1 (Dolby trueHD)) you will not notice the extra seperation in sound. What you will notice is the sound is more defined, and with the right speakers etc you can here clearer sound. Now take all that, run it down a busy free way witll all the road noise, other cars, police, jack hammers etc, and you will likly not notice the extra sound (unless your parked in your garage).
So with that said, 5.1 is by far the most common format, and will serve you very well; I'm not sure how many titles are out but its been very hard for me to find anything encoded past 6.1 for the last few years as my home system is setup for 8.1 sound. with that said, and some very decent speakers for my home system, I can still get better sound from a CD at home, than anything they have shown me with a 7.1 sound system within a car. I have MB ML320CDI with the harmon Kardon system, and its sounds great, but the infiniti 5.1 system sounds better. and again, my home system with a 2 channel audio disc sounds better than either system.
With that said, its mostly marketing at work when you are reading 7.1 today. most cars use a matrix version also, that "up converts" the sounds to the extra channels (see this link)
http://www.harmankardon.com/te...c%207®
And this will take certain "sounds" and ranges of sounds and use its logic (program) to figure out the speakers it thinks it should play them with.
at the end of the day, make up your own mind about it, the links abobve will be healfull, but google is your friend. Most people ona forum like this are "car guys" not "sound guys" so I'm not sure you will get teh best answers from the group of us, if you want to know about sound, hit a sound forum.
There is some of the chips/adds on that can lock doors, and also by pass the warning screen for you, someone posted the link here about 6 months ago. I did a quick search and didnt see it, but I know it was posted as I was going to put the "warning screen bypass chip" just never had the time to buy it, or install it at the time.
tk
Modified by tkeskic at 11:14 PM 1/1/2007