Google has refused to confim how many machines they have, but estimates range between 10,000 and 100,000. Either way, it's many.
I read that to provide page excerpts that a context sensitive to your query they have to keep their entire index (including page contents) in memory. 120GB isn't much when you have that many servers - you could probably keep it in memory a few times over.
With google though, everything's guesswork, because they won't confirm anything.