This is the best set up for something simple but still good...
Get a GOOD headunit don't get an okay one, you want a highend headunit to start off with...( something with good impedance, look for high volt pre outs ( 5 volt + ) and any other options you would like)
Then a good component set ( I got boston rallys and they are a god set for the amount of money you have to spend on them)
- 6.25's in the doors
- tweets in the dash
- make sure they have cross overs
Also you don't want to run these off of deck power, get an amp that will power these, try and get a clean power amp as much as possible, i believe you check this by the THD% rating of the amp or the distortion rating of the amp.........
I personally didn;t even bother with replacing the 5x7's in the back, but I may just throw a new set in and run them off of deck power...
Then for bass, its really up to you what you want, if you want to shake everything in your car loose, or just have some bass to make the music sound good its up to you....
But what i would recommend is doing a custom fiberglass box in the spare tire wheel well, to save on space and it can be made to be stealth, for security reasons...
I went with 3 12's in a custom fiberglass box in the spare tire wheel well, it is a bit much because I have to sound dampen the hatch now....but I would get a top of the line 12 or 15 or even a pair of 12's or 15's and an amp to power them...make sure when your getting the amp for the subs that they are compatible.. if your subs are running at 2 ohm stable and the amp you picked can only run 4 ohm stable...your not going to get the best out of your amp and will probably run in to problems...
The biggest thing I have found for car audio is its hard just to start out with a deck and a few speakers here and keep mixing and matching, and don';t buy cheap **** it will only sound like ass and cost you more money in the end.... make sure the stuff you buy has warranty cuz something is always boudn to **** up...I would personally also get it installed professionally because if something happens, its their grief to fix it not yours...
Also go around town and listen to a whole bunch of different brand names, and see what you like see what fits your budget, then come back on the net and look for product reviews to see if its good stuff or write up another thread on here...and then go buy your stereo.. I wouldn't let the saleman talk you into buying this set of speakers or that set of speakers.,..