I don't know how much I need, but I want the most I can get. I know from experience that I always buy the biggest, or else I regret it later. I bought the biggest safe I could fit into my basement. Last month I bought a 14'x36' shed for my backyard as it was the biggest shed that could fit through my driveway entrance. If there was a legit 3TB enterprise drive like the Western Digital RE4, I would consider upping to 3TBs... I like the fact the the DS1511+ is expandable to add 2 more DX510's.
The system will be used to store all media (HD movies, HD TV shows, music, home movies, pictures) and personal files (word docs, excel, powerpoint, pdfs). I would like to to be able to stream HD video and music - ideally to multiple users simulateously. I would like to be able to access my files over the internet from a remote location.