Just wanted to hop in and say that el duderino's suggestions are very good, and he certainly knows his stuff.
The issue is installation difficulty, etc. That is, a lot of his suggestions might be outside the scope of the average customer, and in many cases, an average installer. So, just wanted to throw out some alternative options for audio upgrades that are much simpler in terms of their installation, which is a better fit for a lot of guys.
X5:
http://www.bavariansoundwerks.com/pr...I=386/_cid=174
http://www.bavariansoundwerks.com/pr...I=363/_cid=196
E39:
http://www.bavariansoundwerks.com/pr...I=380/_cid=174
http://www.bavariansoundwerks.com/pr...I=478/_cid=196
Of course, please voice any questions / concerns.