RCA Adapter for Powerbook?

I don't know if I'm alone, but for the longest time I've wanted some sort of RCA adapter that will allow me to play my xbox, for example, on my powerbook screen. I think I may have seen something like this a long time ago but I'm not even sure. Does this exist, and if so, does anybody have any info on the quality of the feed and the price of something like this? It would be amazingly convenient for me (I have one working screen in my house, I love my powerbook, and I can use it as a portable screen for other things...). Love any input and/or links that can help. Thanks...