Went to see the Roots there yesterday and it was insanely crowded. Also from what I could tell they had standing room only set up (no chairs), which I hope they do for SY as well. You can definitely get a good spot if you want to sit outside the fence or get there late, there's a road that runs behind the fence, also a little hill.
pictures from the Roots:
http://www.brooklynvegan.com/archive...ly_c.html#more
actually I can't tell if there were chairs or not, too many people! haha. I don't think it will get that crowded for SY, but who knows...