Nvidia tests M2 chipsets@ 2006/02/07
For SLI, there is the MCP55P SLI and the MCP55PXE + C51XE positioned as the top dog. The SLI has two 8x slots and 12 more lanes apparently, but the rest is the same as the Ultra. I would guess this is actually the same chip as the Ultra, just fused differently.
The top end XE + C51XE has two 16x slots and 14 more, while adding RAID-5 to the drive capabilities. It is also said to be 'very overclockable'.