Best Steambox for $300-$400

I want to build a good gaming PC to replace my Xbox 360, so I need it to be in the same price range as a console. What are your recommendations as far as the best parts I can get to fit my budget? I won't be building anything for a few months though, cause I don't have any money right now. I want to be able to fit everything in a HTPC case, or some other kind of smaller case. It'll be in my living room and since it is a console replacement, I want it to resemble one in size. I don't need the absolute best hardware to be able to play every game on ultra-high settings. Something that will allow me medium-high on games will be satisfactory. I plan on using Steam's Big Picture mode for this build, that should be great for a living room setup. Keyboard/mouse and TV won't be in this budget, purely the system itself.