What specs should I look for in a HDD solution?

Hey guys. I am looking to setup a NAS for storing media (video and music) that I will pull and play on different computers in the house using XBMC. I'm really not sure what specifications my NAS needs to be built to in order to achieve this as I've just started researching, but I was wondering what type of large storage drive specifications I should look for.

My goal is to start with a 2TB drive and then expand off of it as needed.

While I am not looking for top of the line performance or budget basement, I just want the cheapest available 2TB drive that will allow me to access my video and music data without much buffering so I can enjoy my media as seamlessly as possible.

So what specifications, like RPM, Cache, etc. am I looking for?