My good friend David Yardy posted something about MaxiVista and Windows Vista.  He had a link to a support article from Microsoft discussing multiple monitor support in Windows Vista.

This support article has the following information:

 

Two, three, or more monitors:

•Graphics adapter A with two outputs in an x16 PCIe slot, plus another graphics adapter B with one or more outputs in another x16 PCIe slot.
Both A and B use the same WDDM driver.

•Graphics adapter A with two outputs in an x16 PCIe slot, plus another graphics adapter B with one or more outputs in an x1 PCIe slot.
Both A and B use the same WDDM driver.

•Graphics adapter A with two outputs in an x16 PCIe slot, plus another graphics adapter B with one or more outputs in a conventional PCI slot.
Both A and B use the same WDDM driver.

•Graphics adapter A with two outputs in an AGP slot, plus another graphics adapter B with one or more outputs in a conventional PCI slot.
Both A and B use the same WDDM driver.

•A mobile PC with an internal graphics adapter A, connected to a docking station that includes an external graphics adapter B.
Both the internal graphics adapter A and external graphics adapter B use the same WDDM driver.

Notice that there's nothing listed for two traditional PCI cards.  Maybe this is the piece of the puzzle that I was missing.

I am going to try a PCIe/PCI combination of nVidia cards.  I think this will do the trick after seeing this information from Microsoft.

Thanks Dave!