Well Cortex have developed a card for the Falcon to give it a PCI slot, the new (old) Falcon upgrade from Czuba also has (had) a PCI slot.
Cortex also have a TT upgrade to give it a Faclon upgrade slot and so with the Faclon upgrade a PCI slot.
TT - Falcon slot - PCI slot
There was talks of this card going into the VME slot so that it could be used on the Mega (but only for grahics) - I would prefere the TT mayself then we would also have a PPC upgrade for the TT.
If a VME version of the upgrade is done (and I am tring to keep a difference between the TT and VME version) then I imagine (and I think I may have seen something about it) a VME slot could be put onto an ST so;
ST - VME Slot - Falcon Slot - PCI slot
I would think this would cost around £300 to give your ST a 4 Mb video card, is it worth it??? You also ned drivers and I do not know if fVDI will work.