With car audio, use the KISS theory......
Keep
It
Simple
Stupid!
In reality, all you need a reasonable quality 'brand name' 4 channel amplifier with a low pass filter and around 80 watts RMS to the front channels and around 250 watts RMS + for the rear bridged channels (for the sub). They will advertise this as being around a 600 watt amplifier.
you will need a set of splits, either 5.25", 6.0" or 6.5". JL audio do a fantastic set of 6" splits that JB sell for around $330. You will get much better imaging from splits, but if space doesn't allow, get a good quality set of coaxials.
For the rears, IMHO, leave them stock, as they are just for 'rear fill' and will be powered by the head unit. You really want a loud, crisp and clear front stage and just enough in the rear to entertain passangers sitting in the back.
For a sub, look for something with a reasonable power handling and one that performs well in a small, sealed enclosure (box). The 12" sub is by far the best of both worlds and can give massive amounts of bass. JL Audio also make great value subs..... No, I don't work for JL!
You can go ported, but you will lose boot space and unless you spend heaps, you will also lose sound quality.
This setup should be more than sufficient for most people!
Don't skimp on the amplifier!
Good luck!
Daz.

Muzzy Ti 4 into 1, Power Commander III USB, K&N, Airbox mod, KLEEN delete, Throttle quickener, Vortex -1 front sprocket, Renthal Ultra Low 'bars, Seat cowl, Ohlins 0.95kg fork springs, ZX6R shock. 130hp/ 70ft/lbs!