hey guys
i recently installed some 6"3/4's into my front doors, which obviously really help out. i'd like to boost my sound quality even more though, and am wondering what the most logical course to take would be? I've been told that replacing the rear 6x9's is kinda a waste since they're just "filler noise". is this true?
i was thinking i need a sub/amp but have no idea where to begin on this. thanks in advance!

a new amp that powers the fronts will help a lot, and adding a sub makes a big difference. Its easy to do and all the instructions are in the stereo forum archives

if i'm assuming correctly, that blue.... thing... is a capacitor? will i need one if i'm to install an amp/sub duo? also, can i use the same amp to power the sub and door speakers?

You generally will not need a capacitor unless you are running something like over 500 watts or so (someone correct me if I'm wrong) and cranking it up all the time. The capacitor serves as a place to temporarily store charge. This helps especially if your amps are drawing too much energy from the altenator (causing your lights to dim every time your subs hit).

ahhh it all become clear(er) now! thanks for your help, but i have one last question.... i know everyone has a different view of what a good brand is, but what are your favorites and what sets them apart? thanks again

there is no right answer. Best bet is to go and listen to all the speakers at a local shop. MB Quart and Boston Accoustic, Infinity, and Polk will have a really bright tweeter, JL Audio that I went with is in the middle, Sony, Kenwood, Alpine, Pioneer, Panasonic all tend to be a little flatter. It all comes down to your ear and pocket book