amplifer and 6.75'' speakers

Sorry to break the news to you but $600 on speakers and an amp is not going to get you anywhere near top of the line.  If you're going to just replace the speakers you may want to focus more on your front speakers and get a good soundstage going up front.  Another thing you need to consider is putting in a subwoofer dedicated to producing the low notes your speakers can't reproduce.  Unfortunately $600 just doesn't go real far in the car audio world.

I have an eclipse EA4000 and Polk Momo's 6.5" components in my car.  This system is around $600 retail.  I do like the sound that is produces for the cost.  However, i had Focal in my previous car and Polk's aren't even on the same spectrum in my opinion.  Focal is definately going to be "high end",  running about $300 for entry level speakers to over $1000 for their Utopia line. 

I agree with zhalverson, focus on your front stage (meaning just your front speakers).  If you have a nice set of components up front, the back will matter very little when it comes to listening.  The back speakers are only for the folks riding back there.  Also, if you are seroiusly thinking about having a top of the line system, make sure your CD player is of good quality.  Eclipse or oh yea eclipse should suffice. 

Good luck- stay away from ebay when it comes to high end gear!

That will definitely improve the sound in your car.  Keep in mind the install makes a big difference with speakers.  You are going to want to "deaden" your door's sheet metal to get the speakers to perform well.  I would personally focus more on my frount soundstage and spend a little more on some components up front and either downgrade or completely eliminate the rear speakers.  Your choice though, and if you have rear passengers often they'll benefit. 

Another thing to remember is that those speakers are not going to be able to give you any kind of real bass (100 hz and below).  Often people will try to compensate by turning up the bass on the head unit or amp and drive the speakers into clipping and sometimes failure.  That amp is rated at 150 W per channel and the speakers are 100 W so you'll have to be very careful anyways.  A subwoofer driven off an amp would solve this problem.  Just make sure you know what you want to achieve and get it right the first time and you'll save money in the long run.

$600 is a perfectly reasonable number to work with when replacing the cabin speakers.  He needn't spend $400 on what he is trying to do.

Those are certainly competent choices.  I especially like those new PDX amps ... those are really sexy.

Since you do not indicate a subwoofer, I would probably choose a less expensive cabin amp to match up with some quality coaxials.  Here, I would also recommend you consider Boston Acoustics S-series coaxials... my personal favorite 2-way on the market today.  Many people would say that Alpine's speakers are really screechy and those coaxials you picked ... I think ... are probably among the poorer choices you could make for the money... even at $140 for the pair or whatever.

Unless you are purchasing that PDX to fullfill a size-requirement, it is seriously waaayyy more than you need for what you are doing.  It is however, very, very good looking.

I agree.  $600 will do fine for replacing speakers and an amp.   But to say spending $600 will get you "top of the line" is not really accurate.  Just making a point.  Add a subwoofer, which I think is pretty much a necessity, and you're over $600 before ya know it, not to mention wiring and headunit.