If Richardson falls, take him. If Mathews falls, take him. Otherwise, you will have a shot at BJGE/McGahee type RBs at the 4/5 turn. It's not sexy, but it works. Last season, I didn't pick a RB until the 4th round, and I landed Benson, McGahee and Green-Ellis. They weren't anything to brag about, but they got along just fine with my top WRs (not to mention that I had so much roster turnover that half of my team was cut and replaced by waiver transactions by mid-season, anyway).
After you've taken Foster, you don't have to reach for a RB if it's not good for you. Take what's there, what the draft board gives you, and don't force it or worry about picks that haven't been made, yet. If the RBs are picked clean, then the value is at QB or WR or TE (Gronk/Graham).