A.f.a.i.k these don't exist for socket 775. AMD has some (don't know which though) aswell as Intel but only (correct me if I'm wrong) for XEON's.
Honestly you are not going to need more than one quadcore anyway
As for 2x SLI... do you mean regular SLI or quad SLI because again, you won't need more than regular. 2 GTX's will maim and destroy anything you throw at it exept for VISTA
as the drivers aren't really feeling to good right now. Just imagine the shape of the quad-SLI drivers. Don't overbuy man... it'll be outdated in a few months anyway