in my experience, overflows made NO sense at all to me until I actually hooked it up and figured out how it worked by watching it. If you have a friend that's running a sump/overflow setup, get him to show you how it all works. That'll be MUCH easier to understand than reading about it.
when you're doing your return plumbing REMEMBER TO DRILL A SMALL HOLE just above the water level so if the power goes out, it doesn't syphon the water outta your tank thru your returns. I learned that one the hard way
