sorry I'm not sure about the answer for this..
The standard output on the pc-fx is either composite (3 RCA cables) or S-Video.
I asked some people about the +5v output near the video output on the pc-fx and they said that that is usually for monitor hookup (RGB arcade most likely).. It does seem there is some way to hook the pc-fx up for better video.
Hudsonsoft was able to get HDTV output out of the pc-fx for their unreleased hi-ten bomberman!
there's a website that has pinouts and all kinds of information for this but last time I checked there was no info on pc-fx.. actually they didn't even have the saturn pinout which has been widely available in a pdf of saturn dev docs I believe..
I'll check around..
-RUiNa