I think the bottom line is, for $500, you probably won't be blasting
everyone away.
You can pick up a couple reasonably inexpensive 12" subs, you'll want a ported enclosure, and therefore subs that'll work in ported enclosures well.
Since subs are more efficient in larger enclosures, you'll want to get the biggest enclosure you can fit in your car, which means you'll need to purchase a smaller amplifier...
...how much power your subs need to reach their maximum output level is much more a function of the enclosure, than the subs themselves, since the air trapped inside works like a spring - the smaller the enclosure, the more difficult that spring is to compress (and despite common intuition - ported enclosures actually face
greater pressures inside than a sealed enclosure).
As far as "decent" speakers - it's all relative... relative to what you already have, stock, in the car... relative to your own personal experiences.
You can find a set of aftermarket speakers for your car probably for $30... will they be better?
...better than... what?
