1) I haven't heard of a motherboard lossing dual channel support through overclocking but not saying it might not be true though. Can you link your other post and we might be able to see what other replied and get to the bottom of it? I would say, you'll be safe either way with a 890GX/FX motherboard from ASUS or Gigabyte.
2) When the motherboard says 1600 (OC), it means it doesn't support DDR3 1600 at "stock" settings. You'll have to overclock the memory (manually adjust settings basically) to get it to run at DDR3 1600. It shouldn't be an issue. I would just recommend checking the motherboard's QVL or RAM manufacturer for certification of the memory working with the MOBO. This will help ensure they will all play nice together.