If you are trying to get Aero working in Windows Vista then I recommend you make sure you first meet all the Requirements for Aero. If your pretty sure you do then you may like to know that in short you can only use video cards that share the same driver. Mix and matching video cards is a surefire way for Vista to kick you out of the entire Glass experience.
Some clues may be an error message stating “Incompatible display adapter has been disabled”. You can learn all about this issue a MultiMonitor Support and Windows Vista
Now that you have that problem solved, the only thing you have to worry about now is Vista’s supposed 10 native display limit. I would love to test this out to see if it is true as soon as I have the appropriate motherboard that can handle 5 video cards 😉