I think Williams is a decent RB3 this week, with upside.
Looking back, Miami has had a number of toughish rushing matchups. The Miami D has been very solid, though... going against Buffalo, with Henne and company looking bad, Miami is going to run all day.
I like Ronnie Brown more than Ricky, since he gets most of the wildcat runs and has more touches than Ricky overall, but Ricky is good for 15+ touches against the worst run D in the league. His upside is limited, obviously, by the presence of Brown, but you could certainly do worse.
I am actually starting Ronnie Brown for the first time this year, and I am doing it with confidence.