What answers did you want?
Duo and Quad can run as many applications as you want and fit in the system memory., but a faster CPU will run them better.
Why would you play more than one game at a time? Most systems can load several games, but if they need to run in the backgroind, then you want a powerful CPU to prevent lagging.
You can watch Youtube and play games at the same time, but most human beings can't multitask.