Someone think of a tech question that lots of people will want to answer. Oh and we could try an get 100 replies!
Sure...I can get this to 1000 posts. Here we go...
Which presidential candidate will you vote for, and why?
PCI-Express allows for the expansion of the available bandwidth between the card and the mobo. AGP conventions are limited. PCI-E will double the available bandwitdh over standard 8X AGP.
As for dual-cards, with PCI-E it is possible to physically link two graphics cards together to create a dual-graphics-processor system. This isn't a new concept, but PCI-E makes it much easier, and in the future will make it easily available to the general user with money to spend on a second card.
Actually, the top-bottom thing is only one way of working with dual gpu's. Another way is the splitting-up of the frame processing between cards. Instead of having one card running at 60fps, you can have two cards theoretically pump out twice as much (120fps) in the same relative amount of time. You could also produce the same 60fps playback, but have both cards only doing 30 frames each. This reduces load on the individual card, and not only allows for more processing to be done to each frame, but allows for greater speeds as the work can be shared.