Well I am pretty sure that the GTO2 is a faster card, definately if you unlock it. The question of SM 3 is interesting, but I can honestly say the visual difference is not really a huge one, but it is noticeable when compared to SM 2. All new games are using it now however, but I would think its bette rto go with the x800 simply for increased speed. If you think about it, the 6800gs probly isnt fast enough to give you good fps with cranked visuals and SM 3 effecds since it s a slower card, so its advantage of SM 3 may be lost becasue of that.
Also just about all graphics cards come with and adaptor for pci-e power that connects to a normal atx connector.