Need help on a Pocket size open source computer/media streaming device

Could someone help me. I am interested in building a pocket size or a little larger open source computer/media streaming device. I want to use it as a computer with a real OS for internet and playing audio and video files on a TV or computer monitor, and be able to stream to a wired or wireless network not sure what the best and cheapest components would be any help would be appreciated.

                                                                                                                                                                                                                           Thanks Joe