What rims tires fit 2009 X6 with Sport and Active Steering?

i have a nice set of 20" y rims on a sport package x6 (staggard) with active steering, but the snow will be here in a few months and i cannot imagine 20" summer tires are going to be friendly in Boston. does anyone know what i need to look for when moving to a 19" rim for this kind of unique x6 set up? Will 19 x 8.5 work for all 4 or do i have to have a set of 19 x 10 in the back? i am not concerned with sport performance per se, but certainly do not want to mess up the ABS or speed controls. any pointers for a software geek trying to not buy the wrong rims/tires?