Well, if you have an ATI all-in-wonder card, then you should be able to do that. If your card isn't an all-in-wonder, then you might have problems. Is your s-video port an in or out?
Even if your video card does support it, I wouldn't recommend it (unless you have a big LCD monitor or something). It just makes more sence to plug it into a TV, they are bigger, and (in my case), better quality.