You gotta go RB's first. Don't worry about picking up backup RB's before finishing up the rest of your roster. That is, I'd pick up a 3rd and 4th RB before my 3rd WR, in a league where 2 RB's and 3 WR's are started.
Make a list of all the bye weeks, sorted by week, that way when you pick a backup QB and etc, you can be sure you can use him. That will also help you decide between 2 player ranked similarly.