That is a new enough card, but as far as I can tell, there is no PCI version of the 6600. only AGP and PCI-E.
Are you sure it is PCI?
Lack of an AGP or PCI-E slot is really the problem with this system.
There are optimized drivers for the AGP and PCI-E cards, but no joy on the PCI cards or the on-board Intel AGP