In a league where you start 2 rbs, 3 wrs, and a a flex, choices will be very very slim. If you try to fill your flex with a RB, your WR corp becomes weak. So take the best guy available. This is my roster in a league that starts 2rbs, 3wrs and a flex.
QB: Culpepper, Johnson
RB: McCallister, Barber, Garner, Buckhalter, Griffin,
WR: D. Jackson, J. Smith, Burress, Bennett, Morgan, Warrick