Choosing speakers is mostly based on quality of materials and how much power they need to be able to handle. There are minor variations in frequencies put out, but especially for someone like you, that's irrelevant stuff. Make friends with Crutchfield.com. They make shopping very easy. If you tell them your vehicle, they'll show you everything that fits and even stuff that will fit with a little modification (they tell you which is which), and if you order with your vehicle info put in, you get an installation sheet tailored specifically to your model and, probably, some installation accessories for free. They also have reference material on their site that explains all that you want to know and more.
Honestly, if your original speakers aren't broken, just replacing them with aftermarket speakers isn't going to make that big a difference all by itself. If you get a higher powered head unit or, even better, an amp to power aftermarket speakers, things will sound better. If you also install crossovers to block the bass that small speakers can handle and to block the treble that subwoofers can't handle, everything will sound way better.
Don't be afraid of subwoofers. For some reason (idiots who install huge subs cranked to max and nothing else) people seem to think that subwoofers are evil, annoying bass machines. If you want to hear the low frequencies at all, you NEED a subwoofer. The little 5.25", 6.5", and even 6x9" speakers in vehicles physically cannot produce sounds below a relatively mid-range frequency. You can force them to try, but your sound quality will suffer greatly for it. A single 8" or 10" subwoofer should be more than enough for yousince you're not a crazy audiophile, and you'll save money and space that way.
What I'd do:
- Head unit with all the features I want regardless of power output; it will NOT be powering the speakers.
- Some 5.25" speakers or 6.5" speakers/components for the doors. Rated to at least 50 watts RMS.
- Put 5.25" or 6.5" (if possible") in the sound bar. Again, at least 50 watts RMS.
- A decent 5-channel amp with at least 50 watts RMS to the 4 speakers. It's better to have an amp more too much power for the speakers than to have an underpowered amp; you can always turn the gain down on a more powerful amp.
- An 10" subwoofer mounted like this: http://www.cherokeeforum.com/f10/xj-...y-10-a-120210/
- At least bass-blockers for the speakers if not high-pass crossovers, and a low-pass crossover for the subwoofer.
I'm actually doing all of the above with mine exactly as listed. I might
use two amps--a 4-channel for speakers and a separate subwoofer amp--I'm still deciding.