replacement speakers
You can replace the stock speakers with a 4ohm speaker but the resistance of the voice coils will be double what your factory speakers resistance was resulting in 1/2 the amount of watts to go into them.
Your best bet to get it loud is a four channel amp and some component sets/coaxials. Infinity makes a great sounding speaker that is hard to beat for the price.
That depends. Are you going to use an active cross-over or the Passive one's that would come with the components? Do you all ready have a Sub? What's your averall goal?
Right now, unless money is really tight, The best components that rank the Highest on my "bang for the Buck" are the Infinity Kappa Perfects. I've found them for around $200-$250 a set. Brand new. Great sounding set. I've ordered a set for my self. I'll be able to compare them against my Dyna's to get a comparison.
Check out E-bay. And this place.
http://www.etronics.com/listproducts...1A247B236197B5
They are the ones on the bottom of the page.
Yeah ive heard of em
(they are in my car)MB Quart is the best you can buy as far as sound quality goes, of course thats subjected to my opinion.
To make aftermarket speakers really come ALIVE, like i think your looking for, youll need a aftermarket amp.
I think I will go with the infinity kappa series with a new amp. What size speakers will I need? 
