LJ iand Portis are solid choices here, but if I was drafting a start up dynasty I think I would go younger/less mileage and take either MJD or Gore at this point, so I'll limit my comments to just those two players.
MJD > Gore
MJD is as safe a bet to finish top 10 as anyone else on this list, and he's going to be only 23 years old to start the 2008 season.
Even though Gore has touched the ball more often over the last two seasons, MJD has done significantly more with his touches. Gore had 85 more touches in '07, and MJD still managed to finished ahead of him in fantasy points. Also, he doesn't have the injury history or the offensive woes of San Francisco working against him that Gore does. Better yet he only has a 32 year old Fred Taylor (coming off a career year) standing in his way of increased workload. I'll concede that Gore is more likely to hit 300 carries in '08 than MJD, but Fred Taylor isn't going to be averaging over 5YPC again, and MJD will have to start taking more of the RB load as Freddie declines.
MJD does everything well, 3 down RB, pass protection, catches the ball, takes it to the house, enjoys goal line duties and has a propensity for the end zone having already scored over two dozen touchdowns in his first two seasons. I'll predict at least 200 carries for MJD in 2008 and another top 10 finish with 300+ carries by 2009 and no worse than a top 5 finish in fantasy scoring. He is going to be a S-T-U-D for years to come and IMO 2008 will be the beginning of his ascendancy towards elite status.