Nvidia has made the Chipset on the Ultra-D harder to mod to SLI by puttin enamel over the points that need to be connected, although it can still be done according to what I've read. I've also heard some claim the SLI version gets cherry Nvidia chipset that'll clock higher. My SLI mobo has actually seen higher HTT than my Ultra-D - 370 vs.344 IIRC) so maybe there is something to that rumor.