Hi,
Congratulations on ordering the A1200NG!

You can plug in a USB thumb drive, and it will show up in AmiBench or your custom Workbench installation. Very handy for transferring files between computers!
When adding games, demos and applications to the built-in launcher, the easy way is to have the ADF's or WHD-Load packages on an USB thumb drive, and install them from there. They will then be copied over to the computer.
The A1200NG is a lot of fun, so I'm sure you will enjoy it when it arrives!