If you're looking for overall nice sound, and retain your stock HU, the cleansweep (or RF 3sixty, etc.) would be your best option, and connect it to a 5ch amp (cheaper than a 4ch AND a sub amp, and wiring is simpler and cheaper too). Get some decent speakers (infinity kappa's are real nice, but even their reference coax's would be a huge improvement over your stock stuff) and a single sub (10 or 12 depends on your taste). That will give you a nice all around sounding stereo system.
If you are looking to do a cheaper solution, installing a good aftermarket head unit would be the cheapest solution to improve sound quality.
I installed a pioneer AVIC-Z2 (which is not known for being a 'great' stereo in terms of sound quality compared to other head units that focus more on SQ, but much better than stock) in place of the factory storage compartment, and the sound difference is night and day. I am still planning on upgrading the speakers, adding amps, subs, etc. but only because mobile A/V installation is my passion (and profession). I needed NAVI, so I put in the AVIC, but I don't want to install the rest piece-by-piece, I am waiting for the slow season so I can spend the entire day/weekend dynamatting, wiring, fiberglassing, etc. and not have to worry about installing another component later, it will be all done at once.
