BEWARE, although the rewards are there, audio nervosa is an even greater threat than with 2 channel home stereo. With EQ & crossovers a given, a zillion speakers including multiple subwoofers plus their associated powering amps...well you see what I mean! Having said that, a trip to the bookstore will get you started as there are some decent mags. devoted to the "hobby". As you can probably infer I'm a devotee, Good luck, and by the way if you just want a reasonable upgrade for the Bimmer, Bavarian Soundwerks; http://www.bavariansoundwerks.com/ offers drop-in replacements.
Audio Design Systems 255 10th Avenue New York NY, 212-255-8900
These guy did the radar system in my car and did a great job. There a little pricey, but they do a pro. job.
Find a high end shop in your area that carries the Focal line, Focal makes some really nice component sets for automotive use.
You will need to find a great dealer, who has a real understanding of high-end audio. Cars can sound really, really good, but it is very rare to find a car audio dealer that has a clue whatsoever. Most car audio shop owners have never heard a real high-end stereo, and are in the business of making cars boom and bump.
My best advice is that the installation quality is by far the the most important aspect of any car stereo, so make sure you find someone who knows how to make speaker cabinets out of car doors. My best recommendation, is to not commit to a dealer before hearing one of their car installations, and confirming the "good sound" to them, is anything like "good sound" to you.
I went through 3 car dealers, all of whom promised the world, and delivered extremely sub par, bordering on painful installations. I finally found someone on a car audio forum who really knew what he was doing, and was able to build a car audio system that even an audiophile can enjoy.
I spent years chasing this. The biggest thing I learned is that you can't go in to a store, listen to speakers in a board, and have a good idea of what they will sound like in your car. The car acoustics are so awful, anything that sounds good in a store sound room will sound awful in a car. Good car speakers are designed for the car acoustics and will sound thin and screachy in a regular room. Definitely listen to systems that the shop has installed in cars. Also, make sure they understand the kind of sound you want. Most people want tinkle and boom sound. I was in a local shop, and the owner demonstrated his show car system for me. I told him I thought there was too much bass and highs, and the sound was unnatural overall. He said he understood what I meant, twiddled a few switches and knobs, and played the best car system I have ever heard, better than many home systems. If they answer questions by saying "This si what we do, we are the best, leave it with us, you won't have any regerets..." without some indication that they know what you want, just walk away. (This applies to many things, and is why I built my own deck - too many contractors promising much, with no evidence to back it up).
I had two 3 Series before and I installed Focal speakers in the cars. Go for their top models. And I am using Brax power amps for years. Have yet to find anything better. McIintosh do have car power amps. I only like their head units. The problem is the source. It is going to be hard to replace the head unit unless you want to forgo all the functions in your steering wheel. But there are adapters available whereby you can install a better quality head unit and still have the functions on the steering wheel intact. Funny thing is most of the good head-units are Japanese. Most of the better sounding amps are American. Go figure?!
Went through this stage about six years ago. I quit a few years ago and haven't looked back--tradeoffs just weren't worth it.
It is a pain. I did find a rogue installer who was able to tap the 1V output right in the circuit board of the head unit, so I was able to keep my head unit. My car is a 2005 Acrua TL.
I have 3-way DLS Iridium speakers up front.
Left the rear speakers as is.
ADS old-school 12" sub (much nicer than new stuff)
5 Zapco studio series old-school amps
Tru Trechnology 4-way active crossover (no longer made)
Zapco SX-Sl line driver/crossover/equalizer
I went all-in because I did not have good situation for a home system when I did this four years ago. I would not go as extravagant now.
The mids are 6.5's on the DLS's and I could almost get away without a sub. If we worked on the getting the set with 8's in the door - then for sure. They do come with a very nice passive crossover - so one could do a pretty simple one-amp/passive 3-way set up and be very happy.
Focals were out for me. I would never put metal tweets so close to my ears.
The class D amps are definitely worth a look for auto systems these days due their efficiency. A second (sealed) battery in series is also nice as it adds some oomph and acts like big capacitor.
Elitecaraudio used to be a hot forum, but seems to have died down. YOu may need to search for the best current fourm.
I chased the car audio ghost for years. I gave up after installing a $5,000 upgrade to my Mercedes and it sounded worst than the stock system. In my 2007 BMW 530i I had the store install a nice Sub/amp for about $800. Filled in the bottom enough to be ok. Once my home systems started getting to a level that gave me goose bumps I knew the car no matter how much I spent could get me there. If interested I have boxes and boxes of stuff I have put in and taken out over the years ,amps,speakers,head units,subs,ect, I would sell very reasonably
My advice is to have your car 'sound-proofed'. For about $500 you can buy all the materials to fit your car's floor, doors, trunk, hood, and for a little more, your dash and roof. Another $500 and a good auto upholsterer will install it. Just as a good listening room makes all the difference in home audio, a quiet cabin is the best investment you can make in car audio. Not only will it probably double the improvement of any system you install, it will also make your car rides more pleasant in general.
Another piece of advice is to be savvy about hiding any high end amps under seats or in the trunk, using generic speaker grills (or at least removing high end manufacturer labels) and if possible install a new head unit in the glove compartment. If thieves see a high end stereo your car will at some point getting broken into.
Save your money. The time and effort involved is not worth it. Put the money into your home system. I wish I had all the cash I put into car audio systems in the past. If I had bypassed the car audio trip I could have my dream system today.
For the same reason your home rig will most likely only sound as good as your room will allow, so it goes with the install in a car. As someone who has competed at the highest level in this hobby, and currently owns one the most successful vehicles ever, it's all about the proper implementation of quality components that makes or breaks it. Sadly, VERY few shops are capable of doing it right....if high-end sound in an automobile is what you're after. A great install with regards to keeping the pathlengths of the front stage as equal as possible, keeping the car buzz and rattle free....which can give away location cues if not accomplished, and to top it off, a great tune.....and you can get there.
The problem with high end car audio is that eventually some a*****e will break into your car and steal it. I've given up on car audio after having two head units stolen complete with broken windows and trashed dashboards. It's just not worth it.