2nd hdd dedicated to user files so if windows fails i will never lose anything. You can change the path of documents and everything in windows. 1Tb main HDD. 320gb 2nd HDD for docs, pics, music, video, saved games, etc.
i just have 1tb for my user files and randoms stuff, and my 2tb for games, films and music - the 2tb has 7200rpm, 64mb and F4 so its pretty swift compared to my 32mb f2 1tb