El Duderino's suggestions are solid, but you've got to find a decent shop, and that can be tough. For a self install, a lot of our X5 customers will combine our Stage 1 Audio Upgrade (speaker replacement) with a subwoofer solution. That configuration is enough for 95% of our customers. Here are links to those products:
http://www.bavariansoundwerks.com/pr...I=386/_cid=174
http://www.bavariansoundwerks.com/pr...I=363/_cid=196
Give us an idea of exactly what your big beefs are with the audio system in general -- that'll help figure out what the best path for you to take would be.