Yep. As I said, I am not smart enough to do the math, but neither am I dumb enough to accept the word "impossible" as applicable to generating six billion people from eight. Yet even doing it my own stupid, simple way it doesn't take a lot of multiplication, does it?
Unfortunately doing the math the easy way is not representative of the actual possibilities so will give a figure that is unreasonable.
No population the size of Noah's group would increase at a constant rate (many random events and every event, random or not, will have a larger impact), a constant rate can be postulated for extremely large populations only (this is a function of probability, the larger the population the closer it tends to the average).
Further, a small group is more likely to simply go extinct than it is to increase. (explained here) With a severely small population the likelihood of all offspring being a single sex is relatively high (this is a simple application of probability - 1/2n-1). There is also a tendency for small populations through genetic drift to reduce genetic variation, fix recessive genes in the population and increase their expression (an increase in homozygosity).
There was an earlier post in this thread that linked to a computer program and its output which addressed this very question. The program and its output were reviewed by a number of people before it was posted as a POTM at talkorigins. If you are interested in the difficulty of a population to increase (albeit at a steady rate) in the given time period you should read the post.