If you have auto downloads on, your XBox will automatically download any arcade demos. To use this, you must have a hard drive.
I do not know what you mean by "plug-in", but yes. The first way to trade downloads from one Xbox 360 to the other is to put it on the Xbox 360 memory card. Note that this can only hold 512 Megabytes (half of a gigabyte), and is expensive. The other way, the way that I recommend, is to transfer your Xbox Live profile that you bought all your downloads with to another Xbox 360 and then download them all again. (Note that this will, of course, only work if the Xbox 360 you want to transfer all your downloads has Xbox Live.)
Yes.
yes you can put an xbox 360 harddrive into an arcade 360
Yes you can.
You can trade the Xbox 360 for store credit, and with that you can put it towards the new Xbox 360.
just put the game in the xbox 360
You can put any xbox 360 hard drive on any 360 version
put the firmware from your PC to a usb then connect the usb into the xbox360
NEVER PUT A HARDDRIVE ON A XBOX 360 ARCADE you should have bought the xbox 369 pro
jeez...just put the game disc into the xbox 360(CD/DVD driver)
Yes.
Yes, you can.