What are the original (factory) wheels/tires specs that came with your car?
Basically, your new set "rolling diameter" needs to stay within <=3% (preferablly <1%) of the original set, moreover-especially with xDrive-the difference between the rolling diameter of the new Front set compared to the new rear set should optimally stay below 1%.
Here is a good tire size calculator that I have been using for that purpose:
http://tires.about.com/gi/o.htm?zi=1.../tirecalc.html