Honestly i dont see a reason or need to go above xfire 2 7970's. Pick up 2 of the Sapphire Vapor-x 7970's 6gb Ghz editions and proceed to overclock the crap out of them. I only have 1 sapphire vapor-x 7950 and have overclocked it 20% over stock speeds. That being said my friends pc has SLI 2 690's and the fps in games is so frikin awesome it pisses me off to where i want to throw his comp out a window.
There is some reason to go for more then 2 way because once you go 3 way micro stuttering disappears. Although if that is your problem you would probably going for 3 7870/50's.
Other then that super computers?