want to create a entertainment server

I want to get peoples advice I'm looking to start saving up to build a nice server primarily for movies and tv shows. I'm still deciding wether or not to make it a place to save other things like games and music. I want to make sure it works with any thing I plan to purchase in the future which will most likely be some kind of media device. What is the best OS? Hardware specifics etc. Space is my primary concern cause I definitely want heap loads of HD content. I take all suggestions price wise everything I want to consider and really spend my money wisely. Thanks.