S-video and composite are the same thing. They just use different connections, with S-video simply providing slightly better picture quality..
Also.. I don't believe all this hype about ATI image quality. That was definitely true years back, when the GPU race was heating up and TV-out was becoming a big deal for a lot of people.. ATI may even have slightly better TV out clarity for something like S-video.. However I have never seen any difference that I truly noticed.
That being said, with todays connectors like HDMI and and DVI, and component. It doesn't really matter. My bros shuttle hooked up to his LCD TV (which admittedly does function like a PC monitor by nature) looks crystals clear with great image quality. He just uses the onboard Nvidia GPU for his Shuttle PC.
I have hooked up my Geforce 6800 to our regular big screen downstairs, via it's component out converter box, and had no complaints about the picture quality either.. I'm sure I will get the same, if not better results from my newer 8800 GTS 512.
When it comes to S-video, you are pretty much dealing with an analog signal anyway and what becomes important is not so much your GPU brand, but the quality of your cards DAC (digital to analog converter).