What Matt is pointing out with his photo is path length distance and the peculiarities of having speakers face each other. PLD is very important. However, throwing in reflections is an entire different ball game. Reflections cause cancellations all over the place. They also change the stage in huge ways. Proper speaker placement allows you to pin point the setup of a band or singers. It's also dependent on the recording though.
Anyways, PLD is a huge issue in cars since the drivers aren't equidistant or even sharing a common axis in line wtih our ears. Our dashboards pick up a lot of information and act as wave guides too. Almost unanimously, installers will tell you that kick panels are the #1 choice when optimizing PLD. But that doesn't fix all your problems, it's just one aspect. You also have diffraction too. Also, different frequencies interact differently with all the materials that surround them.
You can fight reflections, or try to work with them. A general rule says keep about 6 inches free in all directions of a speaker. There's actually an equation that helps explain frequency interaction with nearby obstacles and how much space is necessary. Anyways, that's where Home Audio and Car Audio go different directions. Home Audio is trying to limit the interaction of the speakers + environment and create an optimal space for the speaker to operate. Car Audio says there's no chance for that. So techniques arise that are at odds with everything Home Audio represents. eg. pointing R-tweeter 6" in front of the drivers nose and vice versa with the L-tweeter and passenger.
Therefore, Car Audio is basically trial and error instigated by theory and concepts others have used.
...Where Home Audio uses EQ to draw curves, Car Audio uses EQ to fight or enhance reflections in order to raise stage height, move a singer's voice, etc. Does CA still try to draw curves?...yes. What's most important tho?...your ears. That's where HA and CA find common ground. They share a single purpose but their strategies are worlds apart.
Now let's get back to the purpose of "stereo". Stereo is L, R, and C. C is found in R and L channels. C = center stage. R and L are obvious. The purpose of stereo is to recreate the stage that is being recorded. So no matter where your speakers are installed they're trying to replicate a stage. So where is the stage in your car?
Do you want your stage in front of you?...yes. Then install speakers in the front. Do you want your stage behind you?...no. Especially if that stage is competing with the one being created up front. So no rear speakers; unless you're instigating "rear fill"
****I'll provide a link later; for now...FORGET REAR FILL!!!!*****
Moving on. In car audio you want your speakers to work efficiently. Also, symmetry is paramount. 6x9's could work well but typically don't because of lacking design. I refuse to have anything to do with them. Some people like them but we're not talking about speakers in the rear for a good reason right now. Stick with a good pair of 5-1/4" or preferably 6-1/2" speakers if you can. Typically a mid is either good at mid-range or mid-bass but not both. Since most people don't have a trained ear I would stick with a mid that's better at mid-bass.
I'll leave speaker selection to Matt. He knows much more than I. However, I say don't spend more than $200 on retail products. Hugely diminishing returns above that imo.
If you can find some PPI 355cs for under $100 shipped I think it's a huge step in the right direction though.
Well I'm gonna finish for now but I think that explains the disposition Matt's model represented. You're definitely asking good questions and you'd be amazed at what only $500 turns into when you're finished with the product.
Quick picks would be:
$250 Alpine CDA-9887
~$100 PPI 355cs speakers
$79 US Acoustics USB4085 amplifier
http://cgi.ebay.com/US-Acoustics-USB-4085-amplifier_W0QQitemZ180360326172QQcmdZViewItemQQptZCar_Amplifiers?hash=item29fe502c1c&_trksid=p3911.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1|72%3A1205|240%3A1318|301%3A1|293%3A1|294%3A50
Sub and box are another story but they can be fairly cheap too.