If you keep your existing head unit and amp and simply replace the stock speakers with more efficient, higher quality component front door speakers, you will increase overall volume, clarity and definition. You won't get much improvement in the low end as the 5.25" speaker are just too small to produce the lower end of the audio spectrum. If you add any kind of subwoofer, it should be powered as there is not enough power in the OEM amp. As the previous poster said, it all depends on how much you are willing to spend. The sky is the limit when it comes to audio.
From a technical perspective, ohms is a measure of resistance (impedance for speakers). If you replace an 8 ohm speaker with a 4 ohm speaker, it will be more efficient, typically generating a greater volume (sound pressure level, SPL) at the same level of power (watts) applied. So if you want louder sound without changing your amp, go with a speaker with lower resistance (ohms). Decibels is a measure of the loudness and is the units of SPL. Comparing SPL of different speakers is tough because those levels are often given for different power levels and are measured at different distances. Be sure to compare apples to apples. Watts is the measure of power output from your amp. If you increase your power, you can increase your loudness, assuming your speaker can translate that additional power into greater loudness without distortion. Some can't. Higher quality ones can.
Unless you have deep pockets or are a serious audiophile, you are probably more inclined to do incremental improvements vs. a complete system upgrade. I would recommend you first replace the front door speakers with higher quality, more efficient (lower impedance) component speakers. Once you've done that, listen and determine if you want more bass. Then consider the various options for a subwoofer.
Last edited by Fudman; 03-31-2010 at 04:38 AM.