Here is a link (it's actually a sticky), to winter wheel/tire info:
Ben at Tire Rack is a great help.
One recommendation, though - do NOT go with "all season" tires - the only thing "all-season" about them, is that they are equally bad, in ALL seasons. You are MUCH better off, going with a summer tire, and a dedicated winter tire.
If you are going with the OEM 19" wheels, I think Ben recommends Blizzak LM-25s. We use them on our X5, and they work very well. Sizing: 255/35/19 front, 265/35/19 rear. This is less than ideal, as the fronts are actually WIDER than the stock rubber, whereas driving through snow, it would be better going with narrower rubber. That said, i ran 245 width winter rubber all around with my e46 M3, through Canadian Prairie winters for four years, and it worked great!