Advice needed on amplifiers + subs.

I installed a system in my Versa last summer starting with the head unit. I wanted to capitalize on my mini-xm tuner I already owned, so I went to Crutchfield for advice.
Next, I knew that space was limited for an amp, so I looked for a five-channel amp that fit under the front passenger seat. (Alpine PDX-5).
Factory speakers never satisfy me and components have always worked well for sound staging. Get the most speaker that'll fit, in my case again Alpine. Tweeters were mounted in the "A" pillars using fabricated brackets. The back doors have Alpine two-ways.
For a Sub, I really liked the Kicker Solo-Baric L7. Again space was a concern and I wanted tight base, so I went with a sealed enclosure housing a 12 inch sub.
What you build almost needs to grow like branches on a tree. One component leading to the next in power handling and capacity.
The system I installed in my Versa was by far the most complex since it involved navigation, satellite radio, IPod interface, steering wheel controls, Bluetooth and high-amperage potential. I removed the interior plastic and headliner installing sound deadening material. I also routed all the wires under the carpet and through the door access grommets.
In addition, I installed a grounding kit and replaced the battery with a power cell from Kinetik.
I'm super happy with the results. The sound is unbelievable and it takes the edge off driving an "entry level" car.
Crutchfield isn't cheap and you don't need to buy everything from them, but they can certainly guide you to the path you might want to consider.

Here's a slice of what and where I bought my speakers and amp.
Front speakers:
Alpine type R component

Sub:
Solo Baric L7

Rear speakers:
Alpine two-way

Alpine Amp:
PDX-5

Wire:
Monster Wire

RCA Cables:
Monster Cables