You have so many different options, it is hard to recommend a specific setup. If you're not into winning a contest, don't want to mess with tuning, and just want good solid sound, you're on the right track. The retrosound has good basic tuning functions and can support 5 channel output.
A solid option (entry level) would be a 5 channel amp, front/rear component speakers, and (1) 10" or 12" sub.
Everyone has their favorite brands in audio. I spent quite a bit of time researching and wandering around:
http://www.diymobileaudio.com/forum/
Here's a few that get solid reviews on the forums, at entry level prices when this gets asked:
Amps: NVX JAD900.5 amp $220. Zapco ST-X series ($250-$400). Alpine PDX-V9 $700.
Components, is where you want to do your research and spend your money. It wouldn't be hard to spend about $250+ per pair. JL Audio, Image Dynamics, Focal, Morel, and Audiofrog ($$$$); all have multiple options and budgets.
For subs, there are a LOT to choose from mild to wild. Image Dynamics IDQ10 or IDQ12 gets great reviews in small sealed enclosures without breaking the bank. Dig around on the audio forum, and you will find lots of reviews on many subs.
Hope this helps to get you started.