Would be helpful to have an idea of your overall budget, if you are seeking a m-atx case then buying one with a power supply will solve that problem. I would also suggest an extra GB of RAM and use Windows 7 as your operating system. A wireless keyboard/mouse or controller makes sense with a m- atx which I am assuming you will connect to the TV and use that as your monitor. You would also needs wireless card or Ethernet cable to access the internet.
Please post budget and I will spec up a system for you.