Hi There,
Unfortunately, the equipment alone will set oyu back around $1000 -- with what you want to do.
To be perfectly honest, I think the 3 way speaker system that comes with the X5 is fine. What I did was add a 8" subwoofer in the back and that made all the difference in the world. Unfortunately, it was much more than $1000 (but I had Satellite Radio installed too). The enclosure was made of fiberglass which takes a great deal of labor to mold.
Here is some more information, maybe you can duplicated the setup for a lot less. That'd be great.
http://webpages.charter.net/bmwm5/x5sub/stereo.htm
Good Luck!
-Chris