helping a friend build new machine

[insanepsycho]

I'm currently trying to help my friend build a new machine he already has a case, power supply, and monitor. His budget is about $650 but so far i've put together a few things but its over his budget. Anyone have any ideas. He wants to use the machine for casual gaming and hardcore music video streaming/downloading and running things like adobe smoothly.

 

[Muffin Man]

would he be needing a mouse, keyboard, OS also? or just the rest of the hardware going in the tower?

do you think you could list like the model of the case and power supply he already has just to make sure there isnt some kind of restriction, like if the case were to only hold matx boards or a weak psu or something?