A couple months ago I connected an AGP 7600GS 256MB card to a TV using SVideo. It worked, showing the BIOS boot screens and all. I don't think I had any functional problems with SVideo. I had problems with component, which is what I was trying to use. It seemed impossible to output low resolutions with component, but SVideo was able to run at 320x240.
For a moment, I had gotten component to output a 640x480 480i signal to the SDTV which it was able to display. That was the furthest I got with it. However, it was easy to mess this up when I fiddled with things further. I kept ending up in a situation where I needed a computer monitor to see the screen and fix something. SVideo was easier to deal with. NVidia's drivers seem to assume that Component is for an HDTV and SVideo is for an SDTV. I think SVideo always output a usable signal to the TV - I don't think it will even allow you to output a signal on SVideo that an SDTV can't handle.
Bottom line, SVideo seems to work fine with these cards. Component to an SDTV is frustrating.
I had an issue with brightly glowing horizontal lines on the TV. It happened on lines where there was text, or where the white mouse pointer appeared. It was worst with component, but the effect was blurred out some with SVideo. It was noticeable on the GUI desktop, but I don't think I saw it in the NES emulator. I didn't try any modern 3D-era games on it. The issue might have only been at 640x480 and not at 320x240, but I don't remember for sure.
The card I tried above was a 7600GS 256MB AGP. I don't know how it would perform in your target application though, and the RAM is smaller than you're looking for. I think many of the 7800GS cards have 512MB and would be faster, but those cards are usually expensive. They also don't have the best reliability record so you'd need to be careful about not ending up with a bad card.
All the 7000-series NVidias are bridged Express->AGP cards. I don't know if it really matters, I think such cards were tested to work with later era AGP motherboards like you have.
The ATI cards mentioned earlier might be better, I have no idea about that. ATI was usually better than NVidia with home theater type stuff, I wouldn't be surprised if their TV-out support/quality is better.