That would be the Radius SpigotPro AV aka SpigotPower AV then - it has a connector which allows it to access the Quadra's onboard video hardware and DSP, and add its own DSP grunt to the mix. Interestingly, I found a note that it will also work in AV Nubus PowerMacs. The thread where I found that may be of use to you.
IIRC, there are other Nubus cards which don't depend on the Quadra's special hardware and thus can be used in any Mac. I don't recall their names alas. However, if you already have a Q840, then the SpigotPro/Power is probably the one to look for.
NB try using the existing AV system first and see how you like the results. They may be enough for you. You can always look for a Spigot afterwards.