Are you talking about trying to use a motherboard you already have or are you planning to buy a new one? I think you can still find some motherboards based on last year's 690G chipset that come with S-Video out. If it is for an existing MB, you can buy a VGA to S-Video cable for < $10, but your video driver needs to support NTSC output for this to work. A more universal solution would be something like
this, but for the same or less money you could just add an
older discrete video card with S-Video out and have a more elegant solution.