Most factory-installed car speakers are made of lightweight paper or other cheap-to-produce materials. Even "premium" factory speakers tend to wear down and distort a lot faster than aftermarket speakers, so even if they sounded good when you bought the car, they've probably dropped off a bit over the years.
A new set of aftermarket speakers will improve the entire range of your sound, from deeper bass to more well-defined highs. Best of all, they're easy to install and usually more affordable than a receiver.
Aftermarket speakers are made with higher-quality materials, so they'll give you much better sound and increased durability. Full-range speakers (some combination of a woofer, a midrange, and/or a tweeter) fit easily into your car's factory locations and are probably your best bet for improved sound. If you want to hear everything your music has to offer, use a set of component speakers with separate woofers and tweeters to transform your dashboard into a concert soundstage.
If your sound seems to lack that certain something, you need to give your speakers more power. An amplifier increases the volume of your system without distorting it, so you'll have no trouble hearing your tunes with the windows down and the sunroof open. Amplifiers also give your music new life by bringing out musical details and clarity, resulting in better sound quality at all volume levels.
Is your music getting serious competition from your car's exhaust note? You should look into sound deadening materials. A little Dynamat in your vehicle will help keep road noise, vibration, and exhaust rumblings from interfering with your musical enjoyment.
If you like your sound, but want some added depth, a subwoofer is a great addition to any system. Augmenting your setup with a subwoofer driven by an appropriate amplifier will fill in the low notes that small full-range speakers aren't designed to hit, giving you a richer overall sound. You can choose from component subs to customize your sound or preloaded enclosures for easy installation.
If you want a sub, but are concerned about how much space it might take up in your car, then you might want a powered subwoofer or a vehicle-specific subwoofer enclosure. These subs mount in your cargo area or under a seat, so you don't lose much storage space