My opinion is you can't replace the OEM wheels without spending a LOT of money if you want quality and weight. For me, I went with a dedicated winter set up as inexpensive options are plentiful. They are not the same quality as the stock wheels but don't need to be.
So...
1. Buy winter wheels in a square set up which allows rotation and easier to find tires.
2. Winter wheels can be less expensive and that's OK.
3. Replacing OEM wheels with summer wheels will be REALLY expensive if you want similar quality.
4. It makes no sense, to me, to have inexpensive summers and use the forged, light and strong OEM wheels in the winter.
Here is a link to a thread on the direction I went...
http://f80.bimmerpost.com/forums/sho....php?t=1037506
You can get a set of 19" wheels with good winter tires to your door for under $3,000 no problem at all. To replace the OEM summers with similar quality would be $5,000+ for just wheels IMO. Makes no sense UNLESS you don't like the design of the 19" OEM.