After-market car stereo - (First-Gens) - Upgrade and Mod rolled into one really. This isn't for everyone, but I had the base stereo with only aux input, no Bluetooth. Though with ,albeit limited, mp3 support.
If you have the H/K or DPSM, chances are better that you're satisfied with your sound and you already have the Bluetooth module. The Bluetooth module has become a must-have with the hands-free laws in our area.
For those like me that had the base system, after-market is your best option. The head-unit (HU) I chose has:
1. color-matched display (NO WAY did I want blue or green in there)
2. Bluetooth support for hands free calling and music streaming
3. iPod connection/control
4. Aux input
5. mp3 CD support of course (and other digital formats)
6. USB connection (thumbdrives don't skip)
7. steering wheel button integration
8. Sirius/XM add-on support
9. HD Radio add-on support
10. 4-channel line-level outputs for later amp addition
As for sound quality, many folks have debated back and forth on which component swap is more beneficial, some have done only speakers, some have added amps and speakers, some have done the whole kit. In my opinion, the head unit swap is best. Not that it is your last move, but it sets a better stage for the addition of amps and/or speakers at a later date.
...and until I add amps or swap speakers, the HU has a programmable Digital Signal Processor (DSP) that can be adjusted to get the most from the stock speakers - and I really was impressed with the improvement!