I think I've read something interesting about this card (I imagine not many are interested in MPEG2 decoding old solutions but it might be useful info for some users that tested these in an old thread). In a User Manual of the previous generation card (having an older chip model), there's a video card example of... the Riva 128 ( 😀 ) and it says it can go up to 1600x1200 with color depth and monitor freqs listed. Considering they specified the Riva 128 and not the 8MB ZX version, the listed resolutions are the common ones even higher than the one I'm using so video memory may NOT be a problem. At the end the desktop resolutions are probably the limits and it should work on a 4MB card easily. Why it doesn't I'm not sure. The resolutions listed for the Riva 128 are nothing strange, the usual 60, 75 and 85Hz. Color depth 8/16/32 bits depending on the video memory like old 2D desktop limits.
But 800x600@16bit&75Hz is the highest I can run a movie with it even with the monitor disabled Plug&Play to force any resolutions theorically. Also another problem is the driver/sw versions: they can't be mixed and it seems the past companies behind this PCI card changed, who designed it, another who built it as a product, another who took later the project, another who renamed it later and updated its firmware while drivers should anyway work. As usual it's not easy to find all the drivers and sw versions and I might start from this. Anyway the point that it didn't work with anything but the 3dfx card is now explained not really true but depending on multiple factors more on the card drivers/sw version and after that on the video card configuration and time correct drivers.
Update: meanwhile I'm reading the few still complex informations on this card in its lifetime, I think this was interesting. In the last company selling and supporting this card I found this line about the driver/sw installation in its last update:
"For version C3.x [purchased from us on or after 08/Jun/1999] Win98/ME/2000/XP software [will not work on Win95/NT] CineMaster". So I did read somewhere than last cards had a different firmware but it was supposed to be compatible with any drivers. I suppose my own card might be the previous Rev. by Ravisent and not the last updated and renamed while identically built. So if that's true it could mean that latest driver might not always work with all the decoders even with they are identical on the PCB but might differ on the firmware side. Also cause the "feature connector" I never cared reading about as a tech, seems like it can be configured to work in different modes on the vga card. So I don't know if latest video cards having this connectors were configured for a different setting and this might (just a theory) be somehow updated on the decoder (even if it'd sound strange, considering the couple years technology lifetime of these cards).