I installed this last night on my Vista x64 system with aQ6600 and an 8800GT in it. Not only can I run the GPU version but I am able to also run the SMP version on the processor at a slightly lower priority and get 6800PPD on the machine!
Jeff,
That is because the newest nVidia driver that you have to use for this is only made for the GT200 cards. What you have to do is add your cards to an internal file so that it will install the driver properly. This thread will help you if your running Vista x64 as there is a modded .inf file link mid way down for it that will get rid of your error message and let them install with your 8800GT.
http://forums.guru3d.com/showthread.php?t=264622
Keith