It gets ugly. You usually have to install some sort of "WDM Driver" - look on ATi's website for this. At least thats what its called on nVidia setups. The WDM driver lets the card talk to a TV-viewer application like DScaler. (You will definately need this app). Anyway, with my geforce, the picture quality through SVideo was awesome, but unfortunately the video stream was not real-time. It was very laggy, and only updated every couple of frames. VERY bad. I talked to nVidia techs and evga techs, and they official word is that *you cant do that, because its digitizing it, and digitizing it takes time, and therefore there will be a lag*..... Which sounded like bullshit to me but they weren't willing to go any further than that. IMHO its a much better idea to get a dedicated TV Tuner. The only thing I have ever convincingly seen an AIW card do is tune TV channels. I've never seen one pipe through S-Video or Composite in real time, but then again that doesnt really mean it wont work, I've just never seen it before.
Anyway, get dscaler, or try the ATI Multimedia Center thats on the driver download page under All-In-Wonder.
Good luck
~MiSfit