IMO, 120g heavily loaded display (btw, I consider Cody's tank moderately loaded) is the max i would feel comforatable with that skimmer. That being said if I was building a 120, I would choose a 1050 because with heavy bio loads come heavy feedings. I want that extra power just in case.
There are some people that will claim you can run a 180g with the 840. I'm just not one of them!