wow thats a lot of convo.. it hurts my head to read gussies posts sometimes (sorry dude.. i know you have some good insights.. just tough to read)..

I too am a rockford fosgate guy.. ive done like 3 sound systems in my VX so far.. and looking for a 4th so heres my input..

you need a subwoofer.. no way around it.. the vx speaker set up is not good for bass.. gotta add something in somewhere

front doors are 5-3/4 i believe.. i fit small 6-1/2s in there (they were more like 6) with no issues.. then i upgraded to 6-1/2" components with good sized midrange for the door.. i had to do some cutting of the brackets in the doors.. wasnt too bad.. components definitely made a difference in the sound quality but were kind of a pain to do (tweeters were hardest)

rears are 4".. they suck to get to.. dont go halfway on the rears.. when you do them.. go all out, replace the wires etc, you won't want to go back in there once its sealed up again. i think i lost my wheel lock key back near those speakers.. im just paying the $20 to get a new one.... now stock is 4" i cut the mounts and fit shallow mount 6-1/2" speakers back there.. that helped some.. but now i have a 5.1 processor so the rears dont really do much anyway

the best sound quality mod i did was installing an amplifier to all of the speakers, you need a head unit that will support this to do it (ie RCA outputs for each speaker) but now i can blow most people out of the water with no distortion at all. I have each speaker amplified to 50w RMS at 4 ohms

im sure pbkid will confirm this... at least 50% of your sound quality comes from a quality installation. make sure your speakers are mounted solidly, make sure your wires are separated to reduce interference, get high quality signal wires, pay attention to the manufacturers specifications!!!

brands ive used...
rockford fosgate - good speakers, good price.. 8" rockford subs in custom box.. are LOUD especially for 8s.. also using a rockford fosgate head unit.. sound quality is amazing.. love the way it looks, very easy to use.. but hard to find.. they stopped making them years ago
Infinity - my amp powering everything is infinity.. sounds really nice.. a little big.. but very very good
Boss Audio - used their amps at first.. great price.. great size.. sound was OK but kind of "dirty" definitely some interference added by the amp.. volume was great though
Boston Acoustics - used their components in the front.. very nice sounding speakers, the crossovers they came with i didnt like that much, a little tough to work with and i feel like they didnt do a good job, so i got a set of alpine crossovers to replace them.. big thumbs up

there is also a center channel speaker thats available in DIN size from Alpine that i hear is a great addition for an empty din slot