A hitch rack will cause your bikes to get covered with filth... not what you want. Ultimately, a roof rack will give you far more flexibility.

Try going to the Thule rack site http://www.thuleracks.com/thule/default.asp and click on the "Matchmaker". It's an interactive selection tool that will allow you to view available racks on an X5. It will even let you choose the colour of the truck.