an in place upgrade shouldn't destroy your files as long as you follow instructions to only upgrade and not run a clean install.. you should back up your important files and documents prior to upgrading..
i would recommend a clean install anyways to get rid of all that junk you've been accumulating since you had your computer.. in this case, you would already have backed up your files and documents..
btw, that ad says: In-Place Upgrade: Keeps your files, settings, and programs intact from your current version of Windows.
in regards to playing mirror's edge, why wouldn't you be able to play it? (if you do a clean install, you should download the latest drivers for your graphics card)