2.) It can. 8800GTS is PCI-E v1.0 compatible and only has 1 power connector so 150W per card max. That makes 300W theoretical maximum.
You could fit any 1CPU system inside the remaining 300W.
USB offers a max of 0.5A per device so that means a maximum of 2.5W per device or 20W for your devices. Is the 6400+ heavily overclocked?
3.) Not in any way. But you may not care about the difference.