Can you be a bit more descriptive about what you want to stream? ie: is it over the air? is it cable? is it satellite? is it HD or SD? is it copy protected HD material? etc.
There are devices that will allow you to view/record video signals on a Mac - like the Elgato EyeTV hardware or Hauppage (sp?) HDPVR (requires third party software to work), etc. that may be what you're looking for.
Well, you could get a device that would allow you to get the video into your Mac, but I wouldn't want to use it for video games honestly - there would be a lag time on the conversion between what the hardware would receive to what you would see on the screen which would kill playing games. In terms of movies, if they're on DVD, might as well play it directly on the Mac with it's own DVD player software or are referring to movie delivery in a different method?
It'd be nice - but also the bluray would not come out HD - most movies on bluray playback requires HDCP all the way through (from source to final display) to maintain HD, and so even if you did play it back, it'd end up as SD video at best.
It's actually a fairly common question around here - a lot of people want to be able to use their Macbook or iMac screen as a "monitor" for other video sources - it's really just not designed for that style of use