Who should I start week 4?

which 2 rb should i start? adrian peterson against the packers, willis mcgahee against the browns, and marion barber against the rams. also, which 3 wr should i start? roy williams against the bears, javon walker against the colts, brandon marshall against the colts, calvin johnson (questionable for week 4) against the bears, and shaun mcdonald against the bears.