from (http://en.wikipedia.org/wiki/Narus_(company)) we have the following "A single NarusInsight machine can monitor traffic equal to the maximum capacity (10 Gbit/s) of around 39,000 DSL lines or 195,000 telephone modems. But, in practical terms, since individual internet connections are not continually filled to capacity, the 10 Gbit/s capacity of one NarusInsight installation enables it to monitor the combined traffic of several million broadband users."
also if you have not already read this (http://www.wired.com/science/discoveries/news/2006/05/70908) earlier, doesn't hurt to read once again.