Yes X48 is for crossfire, as well as P45. Any nVidia boards such as 790 are for SLI. SLI boards from nVidia are not as stable and not as good overclockers as Intel chipsets.
The new ATI cards blow nVidia out of the water. There is no reason to go SLI right now, unless you have an insane amount of cash to spend and want to SLI GTX 280s.