The November/December, 2004 issue of Archaeology magazine has a very interesting article
on space archaeology, which is online here:

http://www.archaeology.org/0411/etc/space.html

Below is a quote from part of the article:

If one accepts the idea of archaeological research on sites from the history of human exploration in space, it is hardly a giant leap to consider the potential for archaeological fieldwork on the evidence of extraterrestrial civilizations. The late biochemist and science fiction writer Isaac Asimov once speculated that the galaxy may contain 325 million planets with traces of civilizations in ruins. Perhaps our astronomers and their SETI stations are hearing only static through their radio telescopes because they are, in effect, listening for a message from the extraterrestrial equivalent of the ancient Maya or the Sumerians--dead civilizations that can speak to us now only through archaeology. Constructing a catalog of visual signatures of advanced civilizations will someday be within the province of aerospace archaeology. And with a potential cultural resource database of 325 million planets with civilizations in ruins, there sure is a lot of fieldwork to do "out there."

Anyone know where Asimov came up with that number and how and where he published it? Thanks.