Looking for advice here.
I've got a laptop and a desktop, and have been playing everything on my laptop for the last year. Now I'd like to start playing games on my desktop again, too, including Sacred. And I'd like to transfer my characters over when I do that, on a USB key.
What files do I need to transfer and what do I need to do in general so that I can switch between computers as often as I like without losing characters, items, or map and quest progress?

If you are simply continuing an existing game on a different machine, then all you need to do is copy the C:\Program Files\GOG.com\Sacred Gold\save directory from one copy of the game to another. If you want to use you current character in a new game, then you just need to export it first (load up a save game, hit ESC, choose Export), then copy that save directory over.

Sorry I'm at work and can't look to see precisely what you mean by "copy that save directory over." What save directory? Is there a directory called "export" or something? Sorry if this sounds obtuse.

1- On the laptop where you've been playing, go inside the game folder and find a folder called 'save'
2- Copy that folder to your USB key
3-On the desktop where you now want to play, go inside the game folder and paste the 'save' folder you stored on your USB key.
This will allow you to continue playing on the desktop the game you had going on your laptop.
Export is not a folder or directory. You'll find the option to Export a character in the game. Exporting a character means that later on you'll be able to use that character when starting a new game if you want to.

And just to expand on that, when you export a character, the necessary exported files are placed in the same "save" directory that your normal saved games are stored in, so no matter what you are doing, you only have to copy that one directory on to the USB drive.