^ FALSE (to the guy saying you cannot run different cards at the same time
You can run different graphics cards at the same time, just in in crossfire or SLI. An example of this is me using a 7850 for my main display and my integrated intel graphics for my second display at the same time.
As far as windows is concerned, each graphics card is an add on device that requires a driver. As long as the drivers are present, the cards will work.
I dont know why someone would want to do this unless they are running something like Folding@Home where any GPU will work.
This assumes that the drivers play nice together, which they might not. Its certainly not a limitation given by the operating system.