I'm all for looking at potential in the 8-16 range of a dynasty draft, but I'm with OZ, taking a flier on an RB with questionable size/skills etc over an IDP like Mario Williams or AJ Hawk in the late first/early second is usually a mistake.
I think those two are safe bets to end up in a position to be an every week starter for you. However, if a guy like Ernie Sims ends up going to MIN as the WLB in their new Cover-2 defense, though, you can bump him onto that short list as well.
Admittedly, I'm a little more likely to "reach" for these guys as high as the late first of a 12 team league. You need to have some idea where your leaguemates value these characters if you target them as must haves in the second tier of your draft.
For example, I needed to move up to 13 in one, but only 19 in another to get Odell Thurman, who I felt was a safer bet within the Maurice Clarett/Matt Jones/Ryan Moats tier of second level offensive players.