The quick answer is more power, more speakers, and more audio control.

The DSP amp has 14 channel outputs. The X5 with DSP has 12 speakers. There are 3 speakers in each front door, two speakers in each rear door, and two dual-coil subwoofer speakers in the trunk. The amp puts out about 330W total power.

The control for the DSP amp gives you a graphic equalizer plus room size & echo settings. It is a big jump up from the standard stereo system. You can do better with an aftermarket setup, but it will cost a ton more money to get the performance and integration with the car.

A few folks have done upgrade projects. It is not that bad, if you are comfortable doing auto audio projects.

__________________
SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input