Are you going to build it yourself or do you want one already made? It would probably be cheaper to do it yourself, but you could buy a prebuilt one that would do all that for around $500, probably. The problem with the prebuilt ones is they often come with large HDDs, which you won't need just for a few games and to stream TV, but they usually don't come with a dedicated graphics card. Something like
this would probably suit you pretty well. Azn probably knows more about this, but games like Saints Row don't exactly have high hardware specs anyway, so it's possible newer integrated graphics would be fine anyway.