If it uses a seperate PSU I'm sure it's an external one that you plug into the back of the card (where the VGA/DVI plugs), kind of like the external PSUs on consoles (GC, slimline PS2, and the 360).
If it's not going to be 512-bit it will most likely be 384-bit like the G80.
That is one monster of a card and with that much power and the supposed specs I would expect that card to post benchmark scores that are twice that of anything on the market today..........I cant even imagine using 1250W just to run my PC thats more than everything else in my house running at the same time