Julius Jones - is a starter, is supposed to get more red zone touches this year. shares time with marion barber who scored 16 touchdowns last year.
LaMont Jordan - I don't like him, and no one likes the Raiders line, but he's the sole back with Dominic Rhodes suspended 4 games.
Adrian Peterson - the most potential in the group, but right now the starting job is chester taylor's
Ladell Betts - relegated to backup/shared time duty with Portis healthy
Tough choice, probably depends on the matchups. I'd say later in the season, Peterson would be the way to go, once he takes the starting job from Taylor...but for now, I'd probably go with Jones or Jordan, depending on the opponent.
edit: I didn't read that you need to start two more. That makes it easier for me to say then...go with Jones and Jordan, later in the season, probably Peterson and Jones/Jordan.