I agree that you really can't over-skim a mature tank with a good bioload. But with the high powered skimmers on the market these days you really can have too much skimmer for a given system. Example: if you place a skimmer with a 6" neck on a 90 gal system you will have poor results due to the fact that there is not enough organics within the skimmer at a given time to create a stable head of foam to crown into the cup. I am a firm believer of matching skimmers up to their systems and their owners....QUOTE]
Now that is some *good* advice, and after owning some very nice cone skimmers, it's totally accurate.
Some people end up unhappy with their high powered skimmers because they really don't have enough organics to get a good stable head of foam. I've experienced this myself.
Anyway, given the total volume of your system, I would recommend the ATB 1050, especially if the bioload for that system will be heavy. If the bioload is on the light side given the amount of water you have, then I wouldn't be surprised if the ATB 840 would work. I'm sure the 1050 would be fine.