It all depends on how you have the games installed. Actually playing games will not likely overload the connection. Problem comes if someone installs a new game or if some large patch from something like steam gets forced to a machine. A single machine can use 100mbps easily downloading games. When you consider some games are over 50gbyte it can degrade the connection for a long time.
What you need to figure out how to do is prevent downloads from degrading the other pc. Many things you can load on the end pc to control this but it is something you need to think about.