While the numbers are from older machines, they should still be helpful.
In 1999, testing I saw on SINGLE processor systems were reporting 1500 static web pages/second for machines in the 733Mhz range. (PHP generated pages would of course be less [about 25-50% less].) In theory (and yes - I know that things don't scale lineraly), a dual processor system should then be about 3000 static web pages/second. If we presume that today's machines can handle twice the load - you are talking about 6000 static web pages/second/machine.
So, for 100,000 web pages/second - you would need AT LEAST 17 servers (and more then likely more than that so that you could have N+1 redundency, etc.)
There are some real good articles at http://activeanswers.compaq.com
that describe capacity planning for web servers (look under the DISA section). And they even have a sizer available.