Ok I see. So I guess my first two questions were answered.
Edit: taking into account what Junky said, does ATI+ATI/AMD+AMD equal more power or does the new cancel the old?
It will not matter if you buy AMD or Nvidia. They are compatible with each other. That does not work for Crossover / SLi. What it means is that if your motherboard uses AMD/ATi graphics, it will not affect what type of graphics card you get.