However not selecting 2 RB may prove to be suicide, esp if every other team in the league does choose to keep their 2 RB.
If everyone else is keeping their #2 RB, then nobody would draft a RB in the first couple rounds (generally speaking).
Choosing keepers can be very tough sometimes. I try to factor in everything you mentioned. I look to see who my top players are, try and predict who is going to be kept on which team so I will have a better understanding of who I potentially could get back w/ my 1st pick, and figure out where these players would normally go in a redraft.
If you want an easy answer keep 2 RBs and a WR or QB.
But in your situation Im not sold on Brown at all and think of him as more of a headache than anything else. McNabb and Portis are sure things, but I cant really say who I would keep as a #3 keeper w/o knowing everyones lineups and how they drafted and knowing things said during the season that could be clues to who your competition likes.
I would still probably keep Brown for the reason CC mentioned, but Gates is very close, and if the Chargers franchise Brees, I think it would be nearly impossible not to keep Gates.