Google uses 1000 machines to handle a single search query
Jeff Dean from Google has been giving out a few interesting pieces of information, the most notable is that 1000 machines spring into action every time a search is performed.
Jeff gave several examples of how Google has grown from 1999 to 2009. They have x1000 the number of queries now. They have x1000 the processing power (# machines * speed of the machines). They went from query latency normally under 1000ms to normally under 200ms. And, they dropped the update latency by a factor of x10000, going from months to detect a changed web page and update their search results to just minutes.
Their performance gains are also impressive, now serving pages in under 200ms. Jeff credited the vast majority of that to their switch to holding indexes completely in memory a few years back. While that now means that a thousand machines need to handle each query rather than just a couple dozen, Jeff said it is worth it to make searchers see search results nearly instantaneously.