1. There are 2 ways to run an SLI config on an AMD system, and both would require a motherboard replacement. You can go a step back to an older AM3 Nvidia nForce 980a SLI board, or a step ahead into an AM3+ 990X or 990FX board. Both the 990X and 990FX support SLI, not just the FX. As has been said, AM3+ options would allow for eventually installing a Bulldozer CPU.
2. 850W sounds about right to me for an SLI'd 570 config. Though, there's likely several high-quality 750W units that could pull it off. Ttwo 570's in SLI typically pull around 375-400W of power under normal use. When stressed via Kombustor or Furmark, their power consumption can reach the 500W range.
3. I have a sneaking suspicion that two 570's in SLI would very likely choke a PhII 955. That is, unless you can manage to OC it into the 3.8-4.0 GHz range. Even then, it could still struggle at times.