Re: nvsound support status
HW mixing is limited to soundstorm, non-soundstom boards basicly only contain a audio codec from companies like Realtek. Not sure as I don't have an nforce board yet but I think the main advantage of the nvidia driver is that at the moment it allows you to use spdif which I thought didn't work on alsa yet. (might have changed now though)
Personally I would use the alsa drivers in case you are using a non-soundstorm board.