Most 50" 'monitors' are just rear projection TVs. They usually don't have a tuner, only Composite or S-Video inputs. Hook it up to a tuner (a VCR would work), and you can watch TV all you want.
For monitors that take RGB (Usually 32" and smaller), you can buy a NTSC to RGB decoder that converters Composite or S-Video to RGB. In the end, they both look pretty much the same as a TV that size does, though you can get a better quality picture out of them, since arcade monitors typically have more adjustments.