the 6 billion number, as I understand it, is the peak theoretical amount of *very* simple polygon transforms that the Xenos graphics processor can do per second, if all the sub-processors within Xenos are focused on geometry alone. this does *not* mean Xbox360 can display 6 billion polygons per second or anywhere near that number. It is merely an interesting math fugure that really has no bearing on what Xbox360 games will be doing.
Xenos' peak polygon setup figure is 1/12th as much as that 6 billion figure. which is a small fraction of 6 billion. it is the 500 million figure that is really relavent here. and of that (500M) we will see a fraction of it used in first-generation Xbox360 games. lets say 100M. as Xbox360 gets older, programmers will make more and more use of the max 500 million polygons that Xenos can setup, but probably never 100% of it.
edit: sorry, I re-read the post and I see it is asking how did they arrive at the 6 billion figure, sorry I'm no good at math. it's obviously got something to do with the 48 shader ALUs * 500 MHz but I dont know what other calculations are involved. sorry :|