Just read Techreport’s nVidia GF 7900 review with some information about quad-SLI systems. Quite interesting and also funny reading as they really have jokeful editors. Here are some quotes:

“And we have the juicy details on the King Kong of PC graphics, quad SLI, where not one, not two, but four GeForce 7900 GPUs can pull together to squash all competition and raid your Swiss bank account.”
“There’s nothing like playing some F.E.A.R. on a pair of GeForce 7900 GTXs after a long weekend of yachting, but most of us will probably be more personally interested in the G71’s other assignment, the GeForce 7900 GT.”
“Probably a whole truckload of cash, because quad SLI will initially be available only through PC system builders like Voodoo and Alienware, who like to charge about the price of a Mazda for a well-equipped PC.”