I'm also looking to upgrade my suspension. I'm heavily considering 2 options.
1. Full BMW Performance Suspension
Lots of reviews on this site. Just a few.
http://www.1addicts.com/forums/showthread.php?t=403757
http://www.1addicts.com/forums/showthread.php?t=711513
http://www.1addicts.com/forums/showthread.php?t=466597
2. BMW Performance Springs with Bilstein B6 Shocks/Struts
Good review.
http://www.1addicts.com/forums/showthread.php?t=1116635