Lots of options to do what you want. You really need to read some of the forums below and learn a bit about streaming, media PC's, Network Storage, Networking etc. This should give you loads of info on thhe sort of thhings you want to do. i personally am using a PC in my office connected to my router (internet) via cat5 cable and an old original Xbox (connected via Cat5 to my router)that has been chipped and Xbox Media Centre installed (XBMC is free) this plays anything I throw at it that is stored on my office PC. The only downside is that the Xbox isnt really capable of playing 720p and above. So if you are after igh def then the Xbox isnt really up to it.
Anyway good luck and have a look at the forums below.
Xbox Streaming