250GB/256GB SSD for OS and apps. 3TB+ HDD for videos and other data, plus additional HDD(s) for backup purposes.
Personally I avoid saving things into the 'Documents' folder because third-party apps use it as a dumping ground for their settings and other crap. Just create new folders on the other HDD(s) so that everything is neat and organized. Folders can be added to the 'Favourites' list in File Explorer (née Windows Explorer) for quick access.
In Windows 8.1, you can also create a custom 'Library' containing important files so that File History
will back them up automatically (similar to Time Machine in OS X).