There is some really good advice in this thread, The best I think is that it really depends on what you want your system to sound like. If you are going for SPL you will want to stiffen the flat panels maybe even going as far as welding flat stock to them, but for SQ or even just a good street beater sealing the doors will give you good results.
My friends think I am crazy for spending so much on amps, batteries, wire, subs, and speakers, but what they don't know is that the majority of my work has been in sound deadening, and tuning the cabin gain in my car, that is where the real fun is, and it is what makes the biggest differance.
There is a saying on the car audio forums that is incredibly true in all cases, and that is that a system as a whole is 10% equipment and 90% instalation. Take for example Alan dante, he managed 180DB+ with a single subwoofer, and he did it by tuning the car as a whole, not just stacking power to his equipment, he did that too, but with 1 watt he can get 141DB, that is the power of tuning the car, not the equipment.
(he did 182DB with 74,000W and curently holds the world record, that is most likely loud enough to kill you)
EDIT: I think that you should investigate how you like you front sound stage to sound, the biggest desision is to deside if you prefer a sound stage with the highs and lows at the same height, or if you prefer to raise the sound stage by raising the height of the tweeters, that will be the biggest decision to make, next would be if you prefer a stronger mid range, or high end. If you prefer a strong mid range like I do you would be best off running a three way setup with separate mid-bass, mid-range, and tweeters. If you prefer a stronger high range, you would be good with a two way setup with a mid-bass (6.5") and a set of silk tweeters in the sail panels, either way you will want to seal the doors as best you can, maybe even going as far as making sealed pods in the doors.
This would be a great amp to experiment with as it has a large amount of setting to play with, and can even run a active two way setup in stereo
LINK (also Jacob has the best customer support of any car audio company, if you go to any car audio forum with a question he will be there to answer it personally within a day, really he is an awesome person to deal with and has top notch products to boot.
Another thing to decide is if you like "flavored" sound. by that I me listen to a boss system, if you like the smooth sound you would be good with alpine components in your car, if you prefer the sound of pioneer components then pioneer or CDT would be a good choice. I personally run a set of pioneer 720PRS components up front because I enjoy their crisp sound.
Sorry for the long post.