I've had a 9600 running in a mobo which only supports up to 4x so per se that's not an indication of a problem .. and for HCPC usage the AGP bus speed isn't of any great significance as far as performance is concerned.
IIRC, the incompatibilty started when AGP speeds reached 2x so I'd expect a board that can run at 4x would be no problem .. but you're wise to check.
I don't know what the MBO is, its in a Dell Dimension 4100 (about 3 years old). I will not really need it for games, its mostly for the TV out so I can plug it into my TV to watch recordings from my Nebula DigiTV card and stuff like that. As my TV supports 1080i and things I want to try that out too and see how good a picture I can get. If the 9200 will fit the bill then I don't really have a problem with that as it's cheaper, although I suspect the 9600 is better.
As far as I know there's no difference between the S-video outputs on the 9200 and 9600 .. you'd need to use VGA output at least, probably DVI and a big screen (plasma/projector) before the differences would show and even then it's mainly their MPEG decoders that differ, not video processing AIUI.
Also, if your PC is only 3 years old I think it's highly unlikely it's an old version 1 AGP slot in it ... but if it's a Dell why not ring and ask them, they're usually helpful, if you can quote a tag number they can look up on their records.