1. No idea. ATi's motherboards will support it, don't know about any others - no-one has announced them yet.
2. ATi has claimed that you can either run CF + Physics or single card + Physics.
3. A big fat one - R680 allegedly will dissipate up to 250W, so you're looking at 250W x 2 + 125W for the X1950XT (iirc). That's 650W just for the graphics cand physics.
4. ATi will release them as and when they're done.