DinanX5, I'm looking at
www.tirerack.com on a 2004 X5 4.4i (pretty much the same car as yours) and 20x10.5 all around is fine. Even 21x10.5 all around is fine. However the tire sizes they suggest aren't the same as 315/35/20 all around.
Tirerack says:
20x10.5 = 295/40/20
21x10.5 = 285/35/21