The x48 and the P45 support CF not SLI. If you need SLI then you should have a look at the 680, 750, 790 mobos. The 680 is old and tested, the 790 is the next gen high end chipset from nV and the 750 is somewhat cheaper but still very good. The 680 and the 750 support DDR2 while the 790 DDR3. IMHO you should avoid the 780 as it is known to have problems and it is no new product. It consists of the old 680 plus the nv200 bridge chip.
HTH.