Linux Box

I need a linux that I can just pop in an old computer and install a nintendo emulator easily on.

How about Ubuntu and install one of the many emulators from the software center?

Is it possible to put more than one Linux distros on a Live USB? And if you do, how do you choose which one you want to use at start up? Does it look like the dual-boot screen if you had a dual boot on your computer?

It depends on the size of the usb and if it can handle more then one os.

Most people just slap one distro on their usb though.