Beat me to it on the doors. They will bounce and vibrate like a drum head if you don't put something to dampen them a bit inside them. Of course, the best option is to make a speaker enclosure inside the door itself.
My suggestion:
Your current head unit will work fine for any four basic speakers up to hearing damage levels. I would toss the small fronts and go with a pair of 6-7 inch 3 way Kenwoods. these seem to be the biggest bang for the buck. they make two main lines - get the more expensive set. Last time I checked, the difference was only like $20 between the two. Round speakers sound cleaner and more accurate than oval ones, and 6.5 inch is plenty large.
Get a set of 1 inch riser rings. Then make a plastic or metal box for the back side of the door/inside. Rears need a large cabinet of some kind.
As for the sub - ignore it.(see below)
http://www.kenwoodusa.com/Car_Entertainment/Speakers/7''_Speakers/KFC-C1739ie4 of these would make a sub not required. 30hz is as low as most subs go outside of movie sound effects and the like.(It's basically one note, give or take a bit, above the last note on a piano)
But, honestly, even 4x 6.5s of high quality will nearly make you deaf. The trick is the DIY cabinets for each speaker - they need to be as close to air tight as possible to keep the things from sounding tinny. In fact, with most speakers, the cabinet usually makes a larger impact than the actual drivers.
4.0 always sounds better than 4.1, but it requires large main speakers and a good amp. Subs have always been a method to cheat a bit. In home theater, especially, a sub is good, because people don't have speakers the size of a small fridge. But in a car, there's tons of space in the doors and rear behind the seats to make the speaker boxes. A big cabinet like what was posted would give you enough room to get pretty good low frequencies. A 7 inch woofer will go as low as you need for any rock or jazz or hip-hop or... Larger speakers generally waste space and only offer more air movement(which is annoying IMO in a small enclosed space like a car). You want good bass and not one tone "thump thump thump".
This is why it's better musically to have a small stack of 8 inchers if you play bass as opposed to a chuffy 12-15 inch single driver - the bottom note on a 4 string bass isn't even 40hz!(41.204 hz). A 5 string is 30.868hz, which is as low bass notes go with rock and pop and so on.
If you get 6.5s, which I found fine in my last car as well, get these:
http://www.kenwoodusa.com/Car_Entertainment/Speakers/6.5''_Oversized_n_6.5''_Speakers/KFC-1682ie35hz vs 30hz isn't a huge compromise. Possibly use these for the front if space is tight.
..and *NOT* these (cheaper ones you normally find):
http://www.kenwoodusa.com/Car_Entertainment/Speakers/6.5''_Oversized_n_6.5''_Speakers/KFC-1661SBig difference in the clarity(not good) and just not worth saving $15-20.
I mention these because they would still sound good and don't require totally custom fitting and cutting in many cases(riser rings, covers, cabinets, etc - all normally come with 5-6.5s in mind)
But those 7s are beautiful... 4 of them would only set you back $140, tops.
