In answer to your questions:
1. Yes I save them for further processing. You'd be surprised at how much gold is in them that you can't see. Processing that gold out is not something that a beginner should try because it involves some dangerous materials that if not done right can make ya dead!
2. How would you like that measurement? Buckets, pounds, bags? Magnetic or non-magnetic? I have varying amounts of black sands from different locations and keep them separate until I've finished processing them. This allows me to judge how much "invisible" gold is in each location per pound of black sands. "Invisible" gold is stuff so small it can't even be called "Flour" gold. In some areas the only gold you're going to find is the "Invisible" type because it's so fine. Areas like these are usually not worth the time to work as it takes tons of materials to get even a gram or two. Other areas will have much more and can be very worth the time and effort to process the gold out.
Whenever I run out of regular materials to run and get bored I start processing my black sands. This ranges from "first step" secondary processing like "Shake n Bake" to last stage processing of amalgamation. I'm often amazed at how much more gold I can find just after doing a "Shake N Bake" on so called clean sands.