From what I can tell, adding a GREX to that CyberStorm would be the most efficient PCI solution (being direct to the CS local bus)... I don't know if the Voodoo3 would be much better than the CyberVision or not (I would assume so given the faster GPU and more memory, but what good it does I can't say). Of course there's always the advantage of USB via that speedy PCI bus.
Yes, there are improvements that can be made, but the likelyhood of them ever comming to light is very low.
If I won the Mega Millions... Believe me... There'd be one final upgrade to the ol' A4000D's out there.