My advice is those speakers above are crap. No offense. If I had a 600 dollar budget I would:
a) buy a good cd player for about 150 bucks on ebay. I just got an alpine cda 9805 for about that. good unit.
b) get a decent pair of mb quart or focal separates for the front and mount them in the mdf template that everyone likes. I would go with a psd 6.5 and tweet mb quart setup as you are going to find quart a lot cheaper than focal (in my experience). Budget 160 bucks here for speakers and wood.
c) buy a couple of kicker comp 10s for the trunk. Mount them on a baffle board. ebay has kicker comp 10s all day long for something like 75 bucks a pair shipped. Add in the cost of some 3/4 inch MDF and you are looking at about 85 bucks here tops. If you want two more tens later, that will be easy enough to do. You may find that the two tens are sufficient, however.
d) get a nice 2x50 bridgeable amp for the kickers. You won't need more than that for free airs (assuming it bridges to something like 200 at 4 ohms). This shouldn't be more than 100 bucks. I personally like PPI for the bucks to clean watts ratio.
e) wire the front speakers on deck power until you can afford another amp. When you can I would budget about 160 bucks for a 2x75 or so amp.
f) get rid of your rear speakers. Unhook them. Never listen to them again. If I was on your budget I would just cut some MDF templates and replace the rear speakers with them to fill up the holes. Rear speakers are overrated. You have the tens back there anyway. [img]tongue.gif[/img]
g) Spend the rest of the money on the best speaker wire, power wire, fuse/distro blocks, and RCA cables you can afford. Don't skimp here, although it is tempting to do so on something that will just sit under your carpet. I personally like monster cable a lot. Esoteric Audio is good also. I have used both in my system and am happy. I prefer the Monster stuff, though.
That will probably be around 600, but when you add the other amp for the highs it will be more like 800 (and you will want to).
And don't buy used electronics on ebay, only NIB. I have been burned twice on ebay. Both time buying used electronics that were DOA. Lesson learned.
HTH, I was bored at work today. The bottom line is that I have almost this identical system in my impala and it blows people away. Very cheap, very good sound. Frankly, the kicker 10s can be improved on, but definately not for anything close to that price.