The increase in FSB all depends on how much your RAM is able to overclock with an increase in voltage. You also have to consider motherboard limitations. The booster will only help when the motherboard has plenty of headroom to overclock the FSB, and the RAM overclocks well with increased voltage (BH-5/6, OCZ EB mostly). I tried to get 230 FSB stable with my booster. Before I had only reached 225 stable. I couldn't get 230 to be stable though. I know it wasn't my RAM (BH-5) holding me back. Now that I vtt modded my motherboard, I'm expecting 240-250 FSB depending on my motherboard's limitations.
I'm not getting what you are asking in your second question. You can still use dual channel with the booster. Most people only run a max of 2 sticks of memory. Four sticks will most likely hurt the overclocking ability of the RAM because it's harder to get 4 sticks to sync up. If you are only using 2 sticks, most modern motherboards have 3 or more RAM slots. You can just plug the booster into any empty RAM slot.