The first question you need to ask is what motherboard is it? Since the motherboard runs an AGP card, (the ati radeon 9600) we know a few things. first of all the VOODOOII is seriously inferior to the ATI Radeon 9600; I mean the VOODOOII was king of the game in '98, but the ATI Radeon 9600 came out in like 2002. second, the VOODOOII was a PCI card (note, I said PCI not PCIE); which means it was MUCH slower/worse then anything you'd stick in an AGP slot. Finally, you can't xfire/sli an agp to PCI card.
Setting all of that to the side, the ATI GPU is so far beyond the CPU you'll find that PIII to be a bigger issue then the ATI card.