You can only run 2 different cards if their in the same series, IE 5xxx or 6xxx
The problem is that every different series of card uses a different architecture, 40nm,50nm, ect and the drivers simply don't support 2 of those cards working together
the thing is i know a few people doing it in linux but i can't seem to get it to work in windows 7...... i guess windows just doesn't like multiple drivers?