4 x 25A rails = 100A total, or 1200W if you make the mistake of just adding the rails together. It means each rail can have up to 25A on it before it is overloaded, which does not mean that all can be simultaneously at 25A, because obviously, a 750W PSU isn't going to be putting out 1200W.
It is actually 672W max on the 12V rail, or 56A all together.
It will be able to power crossfired 6950's fine