And then there's astoundingly big. From Greg Linden:

Google reportedly had an estimated 450k machines two months ago and adds machines at roughly 100k per quarter. In 2004, each of these machines had 2-4G of memory, and, two years later, likely are up to 8G standard.

That means that Google can store roughly 500k * 8G = 4 petabytes of data in memory on their cluster.

I'm guessing that their local power company loves them, a lot.

Hat tip Patrick Logan