What can I do with 8-gal air tank?

Beside the concerns mentioned on the rating and condition of the auxiliary tank, it sounds like a significant enough increase in volume that I would not be surprised if the end result turned out to be less than satisfactory. Depending of course on what you want to achieve. As far as running air tools, sure, it's going to give you more volume when you first use it, but it's going to take considerably longer to bring that additional capacity back up to pressure, meaning the pump and motor are going to run longer, possibly going beyond their intended service factors. Meanwhile, you are waiting longer to run the next nail or whatever.