>>212282641
>I'm a pussy and afraid I'll somehow lose all my data even if I back it up.
Disconnect the drive before you partition/install GNU/Linux
>If I backup my files to an external HDD, and install Linux (probably Mint) on my PC, can I simply just copy the files over?
yes
>I know not all games can run on Linux, but what about emulation?
Most emulation will be fine

If you're still paranoid you can backup to the external drive and do a dualboot installation, so you keep both Windows and GNU/Linux on the same system.