If your tired of dealing with cheap quality products and want the best skimmer on the market then PM Scott, (Slief) ^ and ask him which Bubble King Skimmer he recommends. These are hands down the best skimmers available IMO.
I'm not the expert like he is but with Bubble King skimmers you do not need to buy one that is 1.5x or 2x your water volume, you really want to go with what they recommend and like Scott said, your display tank volume not total water volume with Sump.
For ~800 gallons mildly stocked I would say a BK Supermarin 300 with RD3 would be a great skimmer to look into. I've had one and it is huge and the RD3 pump is super quiet and easy to adjust.