A stock 840av is better then a stock 950. Since you seem to have a source for Nubus cards I would think a 950 would be a better choice since you can easily expand it to levels an 840av cannot match.

If you can find a spigot pro or spigot pro av the 840av is pretty good.

There are plenty of DSP cards for 950's.

If you just want audio then an Audiomedia II in either machine is good.