Then you are better off using that, rather than the onboard video, for the simple reason that the HDI adapter is harder to find.
Is it the AV card - with two S-Video ports (round MiniDIN- 8) as well as a monitor port? Or the HPV card, which only has the monitor port? The HPV card is faster.
Kan's 6100 site has loads of useful information - the 6100 is the same architecture as the 8100, just in a smaller case and with no Nubus slots.
BTW ignore the prices on that site - they are w-a-y out of date. Stuff for the 6100 is dirt cheap these days, when you can find it.
I don't like your chances on finding an HDI-45 adapter - the port was only used on three Macs, and that was 15 years ago! The DB-15 Mac to VGA adapter to plug into the PDS card is a much more common part, but even that went out of production years ago when Macs moved to VGA and DVI ports.
Worth a shot. Don't forget to try the Trading Post forum here, too.
Yeah, it can be done. Have a search here and through Google and hardwarebook.com
They still do - I have one in my ebay watch list /facepalm. But criminy they're expensive!