I mean, seriously? Get an X[5/6]M, not much has to be modded there. And if you have a mod itch, get a WRX or some other JDM and go crazy! X3 imho is too classy to be modded...
AC Schnitzer is a tasty visual mod:
http://www.bmwblog.com/2011/07/02/bm...-ac-schnitzer/