After installing Vista Service Pack 1 things seemed to be running pretty well. Both ATI cards were functioning and looking good. But after a couple hours of use, system performance degraded, especially relating to video. Then the ATI drivers crashed and restarted successfully, just like the nVidia drivers did.
So, back to 3GB RAM. I will submit a ticket to Microsoft when I have the energy. Otherwise, I am considering this case closed. I cannot use 4GB RAM and 2 video cards on this Dell OptiPlex with Vista 64-bit.