need speaker advice

Something along these lines...
2 sets of these: http://cgi.ebay.com/NEW-Infinity-Ka...39:1|66:2|65:12|240:1318&_trksid=p3286.c0.m14
And one of these: http://cgi.ebay.com/INFINITY-BASSLI...39:1|66:2|65:12|240:1318&_trksid=p3286.c0.m14
A little above your price range, but about the best bang for your buck you can find and the sub/ amp setup will remove easily! This is the type of setup i would recommend for your application. I am an audiophile and a 1st class installer, my opinion is based on many years of dealing with and listening to many different components and setups. My .02

could he get into the basslink II with the add on 4ch amp and a good set of front components? honestly why bother with rear speakers?
http://www.sonicelectronix.com/sear...all&keyword=basslink+II&Submit.x=0&Submit.y=0

could run active on the fronts... maybe a nice set of 6 1/2's? this would sound great, eliminate rear speakers (less weight ),and cost about the same.

i prefer that kind of set-up (2 fronts and sub). rear speakers aren't entirely necessary especially in a mustang (who the heck sits in the back anyway).
if i were to do this myself, i would piece the system together. sub, build a box, components, 4 ch amp (2 ch bridged for sub and 2 ch for comps.). mount the amp to the box and pull it out when needed (will have to disconnect everything but should not be too difficult).

lol how about 2 speakers for 500?

Look up Hertz Or Dyna Audio

Yes, running 2 spk and a sub/ subs is s a good thing. My GT is set up that way, I have components in the front and 2 12"s in the hatch.(It's my sound quality/ quantity vehicle.) The 2 speaker setup helps eliminate phasing problems associated with the 4 spk sterio setups. I did'nt want to get too technichal on a basic question, but the old "more is better" theory is out the window on this one. Get some good components and a good amp and install them correctly and you'll swear you're at the concert. (Did I mention that I'm an audiophile? and just can't drive my cars without the sterio on?) You're headed in a good direction, just make sure all the work is done correctly and with sound quality in mind. Do a little reasearch on accoustics and apply youre knowledge of 'em. A little knowledge can make a huge difference in system preformance. Mount the crossovers inside the vehicle and run wires into the doors or use the factory wiring as there is already seperate mid/ tweet wiring run into the doors. (keep the crossovers out of the elements.) Possably mount them where your factory tweet amp was. I am assuming you have the mach system being that you have a Cobra and made mention of the factory tweets. Oh yeah, and as far as pricy components (Previous post.) Focal Kit No6 - Utopia Be 6-1/2" Two-Way Component System

The Quarts are good speakers, go with 'em. I recommended the basslink because of ease of removal. If a seperate amp/ sub setup works for you, go with that, that setup will have better output, but is not as "portable". Go with a name-brand amp/ sub, there are quite a few good companies out there, but remember the system chain is only as strong as (or sounds as good as) it's weakest link, so dont go super cheap. Let us know what you end up with and what you think of it!!!

Look up Hertz or Dyna Audio for Component mains. If you have the money back them up with a Zapco reference amp. Sub wise Hertz makes good subs or if you want something that can handle high amounts of power and burp insane amounts of power then look up Digital Design subs (DD) They are good subs and hit hard and loud!!!